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Rails Internationalization (I18n) API

The Ruby I18n (shorthand for internationalization) gem which is shipped with Ruby on Rails (starting from Rails 2.2) provides an easy-to-use and extensible framework for translating your application to a single custom language other than English or for providing multi-language support in your application.

The process of “internationalization” usually means to abstract all strings and other locale specific bits (such as date or currency formats) out of your application. The process of “localization” means to provide translations and localized formats for these bits. 1

So, in the process of internationalizing your Rails application you have to:

In the process of localizing your application you’ll probably want to do the following three things:

This guide will walk you through the I18n API and contains a tutorial how to internationalize a Rails application from the start.

The Ruby I18n framework provides you with all necessary means for internationalization/localization of your Rails application. You may, however, use any of various plugins and extensions available, which add additional functionality or features. See the Rails I18n Wiki for more information.

1 How I18n in Ruby on Rails Works

Internationalization is a complex problem. Natural languages differ in so many ways (e.g. in pluralization rules) that it is hard to provide tools for solving all problems at once. For that reason the Rails I18n API focuses on:

  • providing support for English and similar languages out of the box
  • making it easy to customize and extend everything for other languages

As part of this solution, every static string in the Rails framework — e.g. Active Record validation messages, time and date formats — has been internationalized, so localization of a Rails application means “over-riding” these defaults.

1.1 The Overall Architecture of the Library

Thus, the Ruby I18n gem is split into two parts:

  • The public API of the i18n framework — a Ruby module with public methods that define how the library works
  • A default backend (which is intentionally named Simple backend) that implements these methods

As a user you should always only access the public methods on the I18n module, but it is useful to know about the capabilities of the backend.

It is possible (or even desirable) to swap the shipped Simple backend with a more powerful one, which would store translation data in a relational database, GetText dictionary, or similar. See section Using different backends below.

1.2 The Public I18n API

The most important methods of the I18n API are:

translate # Lookup text translations
localize  # Localize Date and Time objects to local formats

These have the aliases #t and #l so you can use them like this:

I18n.t 'store.title'

There are also attribute readers and writers for the following attributes:

load_path         # Announce your custom translation files
locale            # Get and set the current locale
default_locale    # Get and set the default locale
exception_handler # Use a different exception_handler
backend           # Use a different backend

So, let’s internationalize a simple Rails application from the ground up in the next chapters!

2 Setup the Rails Application for Internationalization

There are just a few simple steps to get up and running with I18n support for your application.

2.1 Configure the I18n Module

Following the convention over configuration philosophy, Rails will set up your application with reasonable defaults. If you need different settings, you can overwrite them easily.

Rails adds all .rb and .yml files from the config/locales directory to your translations load path, automatically.

The default en.yml locale in this directory contains a sample pair of translation strings:

  hello: "Hello world"

This means, that in the :en locale, the key hello will map to the Hello world string. Every string inside Rails is internationalized in this way, see for instance Active Record validation messages in the activerecord/lib/active_record/locale/en.yml file or time and date formats in the activesupport/lib/active_support/locale/en.yml file. You can use YAML or standard Ruby Hashes to store translations in the default (Simple) backend.

The I18n library will use English as a default locale, i.e. if you don’t set a different locale, :en will be used for looking up translations.

The i18n library takes a pragmatic approach to locale keys (after some discussion), including only the locale (“language”) part, like :en, :pl, not the region part, like :en-US or :en-UK, which are traditionally used for separating “languages” and “regional setting” or “dialects”. Many international applications use only the “language” element of a locale such as :cz, :th or :es (for Czech, Thai and Spanish). However, there are also regional differences within different language groups that may be important. For instance, in the :en-US locale you would have $ as a currency symbol, while in :en-UK, you would have £. Nothing stops you from separating regional and other settings in this way: you just have to provide full “English – United Kingdom” locale in a :en-UK dictionary. Various Rails I18n plugins such as Globalize2 may help you implement it.

The translations load path (I18n.load_path) is just a Ruby Array of paths to your translation files that will be loaded automatically and available in your application. You can pick whatever directory and translation file naming scheme makes sense for you.

The backend will lazy-load these translations when a translation is looked up for the first time. This makes it possible to just swap the backend with something else even after translations have already been announced.

The default application.rb files has instructions on how to add locales from another directory and how to set a different default locale. Just uncomment and edit the specific lines.

# The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
# config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
# config.i18n.default_locale = :de

2.2 Optional: Custom I18n Configuration Setup

For the sake of completeness, let’s mention that if you do not want to use the application.rb file for some reason, you can always wire up things manually, too.

To tell the I18n library where it can find your custom translation files you can specify the load path anywhere in your application – just make sure it gets run before any translations are actually looked up. You might also want to change the default locale. The simplest thing possible is to put the following into an initializer:

# in config/initializers/locale.rb

# tell the I18n library where to find your translations
I18n.load_path += Dir[Rails.root.join('lib', 'locale', '*.{rb,yml}')]

# set default locale to something other than :en
I18n.default_locale = :pt

2.3 Setting and Passing the Locale

If you want to translate your Rails application to a single language other than English (the default locale), you can set I18n.default_locale to your locale in application.rb or an initializer as shown above, and it will persist through the requests.

However, you would probably like to provide support for more locales in your application. In such case, you need to set and pass the locale between requests.

You may be tempted to store the chosen locale in a session or a cookie. Do not do so. The locale should be transparent and a part of the URL. This way you don’t break people’s basic assumptions about the web itself: if you send a URL of some page to a friend, she should see the same page, same content. A fancy word for this would be that you’re being RESTful. Read more about the RESTful approach in Stefan Tilkov’s articles. There may be some exceptions to this rule, which are discussed below.

The setting part is easy. You can set the locale in a before_filter in the ApplicationController like this:

before_filter :set_locale
def set_locale
  # if params[:locale] is nil then I18n.default_locale will be used
  I18n.locale = params[:locale]

This requires you to pass the locale as a URL query parameter as in (This is, for example, Google’s approach.) So http://localhost:3000?locale=pt will load the Portuguese localization, whereas http://localhost:3000?locale=de would load the German localization, and so on. You may skip the next section and head over to the Internationalize your application section, if you want to try things out by manually placing the locale in the URL and reloading the page.

Of course, you probably don’t want to manually include the locale in every URL all over your application, or want the URLs look differently, e.g. the usual versus Let’s discuss the different options you have.

2.4 Setting the Locale from the Domain Name

One option you have is to set the locale from the domain name where your application runs. For example, we want to load the English (or default) locale, and to load the Spanish locale. Thus the top-level domain name is used for locale setting. This has several advantages:

  • The locale is an obvious part of the URL.
  • People intuitively grasp in which language the content will be displayed.
  • It is very trivial to implement in Rails.
  • Search engines seem to like that content in different languages lives at different, inter-linked domains.

You can implement it like this in your ApplicationController:

before_filter :set_locale

def set_locale
  I18n.locale = extract_locale_from_tld

# Get locale from top-level domain or return nil if such locale is not available
# You have to put something like:
# in your /etc/hosts file to try this out locally
def extract_locale_from_tld
  parsed_locale ='.').last
  I18n.available_locales.include?(parsed_locale.to_sym) ? parsed_locale  : nil

We can also set the locale from the subdomain in a very similar way:

# Get locale code from request subdomain (like http://it.application.local:3000)
# You have to put something like:
# gr.application.local
# in your /etc/hosts file to try this out locally
def extract_locale_from_subdomain
  parsed_locale = request.subdomains.first
  I18n.available_locales.include?(parsed_locale.to_sym) ? parsed_locale  : nil

If your application includes a locale switching menu, you would then have something like this in it:

link_to("Deutsch", "#{APP_CONFIG[:deutsch_website_url]}#{request.env['REQUEST_URI']}")

assuming you would set APP_CONFIG[:deutsch_website_url] to some value like

This solution has aforementioned advantages, however, you may not be able or may not want to provide different localizations (“language versions”) on different domains. The most obvious solution would be to include locale code in the URL params (or request path).

2.5 Setting the Locale from the URL Params

The most usual way of setting (and passing) the locale would be to include it in URL params, as we did in the I18n.locale = params[:locale] before_filter in the first example. We would like to have URLs like or in this case.

This approach has almost the same set of advantages as setting the locale from the domain name: namely that it’s RESTful and in accord with the rest of the World Wide Web. It does require a little bit more work to implement, though.

Getting the locale from params and setting it accordingly is not hard; including it in every URL and thus passing it through the requests is. To include an explicit option in every URL (e.g. link_to( books_url(:locale => I18n.locale))) would be tedious and probably impossible, of course.

Rails contains infrastructure for “centralizing dynamic decisions about the URLs” in its ApplicationController#default_url_options, which is useful precisely in this scenario: it enables us to set “defaults” for url_for and helper methods dependent on it (by implementing/overriding this method).

We can include something like this in our ApplicationController then:

# app/controllers/application_controller.rb
def default_url_options(options={})
  logger.debug "default_url_options is passed options: #{options.inspect}\n"
  { :locale => I18n.locale }

Every helper method dependent on url_for (e.g. helpers for named routes like root_path or root_url, resource routes like books_path or books_url, etc.) will now automatically include the locale in the query string, like this: http://localhost:3001/?locale=ja.

You may be satisfied with this. It does impact the readability of URLs, though, when the locale “hangs” at the end of every URL in your application. Moreover, from the architectural standpoint, locale is usually hierarchically above the other parts of the application domain: and URLs should reflect this.

You probably want URLs to look like this: (which loads the English locale) and (which loads the Netherlands locale). This is achievable with the “over-riding default_url_options” strategy from above: you just have to set up your routes with path_prefix option in this way:

# config/routes.rb
scope "/:locale" do
  resources :books

Now, when you call the books_path method you should get "/en/books" (for the default locale). An URL like http://localhost:3001/nl/books should load the Netherlands locale, then, and following calls to books_path should return "/nl/books" (because the locale changed).

If you don’t want to force the use of a locale in your routes you can use an optional path scope (donated by the use brackets) like so:

# config/routes.rb
scope "(:locale)", :locale => /en|nl/ do
  resources :books

With this approach you will not get a Routing Error when accessing your resources such as http://localhost:3001/books without a locale. This is useful for when you want to use the default locale when one is not specified.

Of course, you need to take special care of the root URL (usually “homepage” or “dashboard”) of your application. An URL like http://localhost:3001/nl will not work automatically, because the root :to => "books#index" declaration in your routes.rb doesn’t take locale into account. (And rightly so: there’s only one “root” URL.)

You would probably need to map URLs like these:

# config/routes.rb
match '/:locale' => 'dashboard#index'

Do take special care about the order of your routes, so this route declaration does not “eat” other ones. (You may want to add it directly before the root :to declaration.)

Have a look at two plugins which simplify work with routes in this way: Sven Fuchs’s routing_filter and Raul Murciano’s translate_routes.

2.6 Setting the Locale from the Client Supplied Information

In specific cases, it would make sense to set the locale from client-supplied information, i.e. not from the URL. This information may come for example from the users’ preferred language (set in their browser), can be based on the users’ geographical location inferred from their IP, or users can provide it simply by choosing the locale in your application interface and saving it to their profile. This approach is more suitable for web-based applications or services, not for websites — see the box about sessions, cookies and RESTful architecture above.

2.6.1 Using Accept-Language

One source of client supplied information would be an Accept-Language HTTP header. People may set this in their browser or other clients (such as curl).

A trivial implementation of using an Accept-Language header would be:

def set_locale
  logger.debug "* Accept-Language: #{request.env['HTTP_ACCEPT_LANGUAGE']}"
  I18n.locale = extract_locale_from_accept_language_header
  logger.debug "* Locale set to '#{I18n.locale}'"
def extract_locale_from_accept_language_header

Of course, in a production environment you would need much more robust code, and could use a plugin such as Iain Hecker’s http_accept_language or even Rack middleware such as Ryan Tomayko’s locale.

2.6.2 Using GeoIP (or Similar) Database

Another way of choosing the locale from client information would be to use a database for mapping the client IP to the region, such as GeoIP Lite Country. The mechanics of the code would be very similar to the code above — you would need to query the database for the user’s IP, and look up your preferred locale for the country/region/city returned.

2.6.3 User Profile

You can also provide users of your application with means to set (and possibly over-ride) the locale in your application interface, as well. Again, mechanics for this approach would be very similar to the code above — you’d probably let users choose a locale from a dropdown list and save it to their profile in the database. Then you’d set the locale to this value.

3 Internationalizing your Application

OK! Now you’ve initialized I18n support for your Ruby on Rails application and told it which locale to use and how to preserve it between requests. With that in place, you’re now ready for the really interesting stuff.

Let’s internationalize our application, i.e. abstract every locale-specific parts, and then localize it, i.e. provide necessary translations for these abstracts.

You most probably have something like this in one of your applications:

# config/routes.rb
Yourapp::Application.routes.draw do
  root :to => "home#index"

# app/controllers/home_controller.rb
class HomeController < ApplicationController
  def index
    flash[:notice] = "Hello Flash"

# app/views/home/index.html.erb
<h1>Hello World</h1>
<p><%= flash[:notice] %></p>

rails i18n demo untranslated

3.1 Adding Translations

Obviously there are two strings that are localized to English. In order to internationalize this code, replace these strings with calls to Rails’ #t helper with a key that makes sense for the translation:

# app/controllers/home_controller.rb
class HomeController < ApplicationController
  def index
    flash[:notice] = t(:hello_flash)

# app/views/home/index.html.erb
<h1><%=t :hello_world %></h1>
<p><%= flash[:notice] %></p>

When you now render this view, it will show an error message which tells you that the translations for the keys :hello_world and :hello_flash are missing.

rails i18n demo translation missing

Rails adds a t (translate) helper method to your views so that you do not need to spell out I18n.t all the time. Additionally this helper will catch missing translations and wrap the resulting error message into a <span class="translation_missing">.

So let’s add the missing translations into the dictionary files (i.e. do the “localization” part):

# config/locales/en.yml
  hello_world: Hello world!
  hello_flash: Hello flash!

# config/locales/pirate.yml
  hello_world: Ahoy World
  hello_flash: Ahoy Flash

There you go. Because you haven’t changed the default_locale, I18n will use English. Your application now shows:

rails i18n demo translated to English

And when you change the URL to pass the pirate locale (http://localhost:3000?locale=pirate), you’ll get:

rails i18n demo translated to pirate

You need to restart the server when you add new locale files.

You may use YAML (.yml) or plain Ruby (.rb) files for storing your translations in SimpleStore. YAML is the preferred option among Rails developers. However, it has one big disadvantage. YAML is very sensitive to whitespace and special characters, so the application may not load your dictionary properly. Ruby files will crash your application on first request, so you may easily find what’s wrong. (If you encounter any “weird issues” with YAML dictionaries, try putting the relevant portion of your dictionary into a Ruby file.)

3.2 Adding Date/Time Formats

OK! Now let’s add a timestamp to the view, so we can demo the date/time localization feature as well. To localize the time format you pass the Time object to I18n.l or (preferably) use Rails’ #l helper. You can pick a format by passing the :format option — by default the :default format is used.

# app/views/home/index.html.erb
<h1><%=t :hello_world %></h1>
<p><%= flash[:notice] %></p
<p><%= l, :format => :short %></p>

And in our pirate translations file let’s add a time format (it’s already there in Rails’ defaults for English):

# config/locales/pirate.yml
      short: "arrrround %H'ish"

So that would give you:

rails i18n demo localized time to pirate

Right now you might need to add some more date/time formats in order to make the I18n backend work as expected (at least for the ‘pirate’ locale). Of course, there’s a great chance that somebody already did all the work by translating Rails’ defaults for your locale. See the rails-i18n repository at Github for an archive of various locale files. When you put such file(s) in config/locales/ directory, they will automatically be ready for use.

3.3 Localized Views

Rails 2.3 introduces another convenient localization feature: localized views (templates). Let’s say you have a BooksController in your application. Your index action renders content in app/views/books/index.html.erb template. When you put a localized variant of this template: in the same directory, Rails will render content in this template, when the locale is set to :es. When the locale is set to the default locale, the generic index.html.erb view will be used. (Future Rails versions may well bring this automagic localization to assets in public, etc.)

You can make use of this feature, e.g. when working with a large amount of static content, which would be clumsy to put inside YAML or Ruby dictionaries. Bear in mind, though, that any change you would like to do later to the template must be propagated to all of them.

3.4 Organization of Locale Files

When you are using the default SimpleStore shipped with the i18n library, dictionaries are stored in plain-text files on the disc. Putting translations for all parts of your application in one file per locale could be hard to manage. You can store these files in a hierarchy which makes sense to you.

For example, your config/locales directory could look like this:


This way, you can separate model and model attribute names from text inside views, and all of this from the “defaults” (e.g. date and time formats). Other stores for the i18n library could provide different means of such separation.

The default locale loading mechanism in Rails does not load locale files in nested dictionaries, like we have here. So, for this to work, we must explicitly tell Rails to look further:

# config/application.rb
  config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}')]

Do check the Rails i18n Wiki for list of tools available for managing translations.

4 Overview of the I18n API Features

You should have good understanding of using the i18n library now, knowing all necessary aspects of internationalizing a basic Rails application. In the following chapters, we’ll cover it’s features in more depth.

Covered are features like these:

  • looking up translations
  • interpolating data into translations
  • pluralizing translations
  • localizing dates, numbers, currency, etc.

4.1 Looking up Translations

4.1.1 Basic Lookup, Scopes and Nested Keys

Translations are looked up by keys which can be both Symbols or Strings, so these calls are equivalent:

I18n.t :message
I18n.t 'message'

The translate method also takes a :scope option which can contain one or more additional keys that will be used to specify a “namespace” or scope for a translation key:

I18n.t :record_invalid, :scope => [:activerecord, :errors, :messages]

This looks up the :record_invalid message in the Active Record error messages.

Additionally, both the key and scopes can be specified as dot-separated keys as in:

I18n.translate "activerecord.errors.messages.record_invalid"

Thus the following calls are equivalent:

I18n.t 'activerecord.errors.messages.record_invalid'
I18n.t 'errors.messages.record_invalid', :scope => :active_record
I18n.t :record_invalid, :scope => 'activerecord.errors.messages'
I18n.t :record_invalid, :scope => [:activerecord, :errors, :messages]
4.1.2 Defaults

When a :default option is given, its value will be returned if the translation is missing:

I18n.t :missing, :default => 'Not here'
# => 'Not here'

If the :default value is a Symbol, it will be used as a key and translated. One can provide multiple values as default. The first one that results in a value will be returned.

E.g., the following first tries to translate the key :missing and then the key :also_missing. As both do not yield a result, the string “Not here” will be returned:

I18n.t :missing, :default => [:also_missing, 'Not here']
# => 'Not here'
4.1.3 Bulk and Namespace Lookup

To look up multiple translations at once, an array of keys can be passed:

I18n.t [:odd, :even], :scope => 'activerecord.errors.messages'
# => ["must be odd", "must be even"]

Also, a key can translate to a (potentially nested) hash of grouped translations. E.g., one can receive all Active Record error messages as a Hash with:

I18n.t 'activerecord.errors.messages'
# => { :inclusion => "is not included in the list", :exclusion => ... }
4.1.4 “Lazy” Lookup

Rails 2.3 implements a convenient way to look up the locale inside views. When you have the following dictionary:

      title: "Título"

you can look up the books.index.title value inside app/views/books/index.html.erb template like this (note the dot):

<%= t '.title' %>

4.2 Interpolation

In many cases you want to abstract your translations so that variables can be interpolated into the translation. For this reason the I18n API provides an interpolation feature.

All options besides :default and :scope that are passed to #translate will be interpolated to the translation:

I18n.backend.store_translations :en, :thanks => 'Thanks %{name}!'
I18n.translate :thanks, :name => 'Jeremy'
# => 'Thanks Jeremy!'

If a translation uses :default or :scope as an interpolation variable, an I18n::ReservedInterpolationKey exception is raised. If a translation expects an interpolation variable, but this has not been passed to #translate, an I18n::MissingInterpolationArgument exception is raised.

4.3 Pluralization

In English there are only one singular and one plural form for a given string, e.g. “1 message” and “2 messages”. Other languages (Arabic, Japanese, Russian and many more) have different grammars that have additional or fewer plural forms. Thus, the I18n API provides a flexible pluralization feature.

The :count interpolation variable has a special role in that it both is interpolated to the translation and used to pick a pluralization from the translations according to the pluralization rules defined by CLDR:

I18n.backend.store_translations :en, :inbox => {
  :one => '1 message',
  :other => '%{count} messages'
I18n.translate :inbox, :count => 2
# => '2 messages'

The algorithm for pluralizations in :en is as simple as:

entry[count == 1 ? 0 : 1]

I.e. the translation denoted as :one is regarded as singular, the other is used as plural (including the count being zero).

If the lookup for the key does not return a Hash suitable for pluralization, an 18n::InvalidPluralizationData exception is raised.

4.4 Setting and Passing a Locale

The locale can be either set pseudo-globally to I18n.locale (which uses Thread.current like, e.g., or can be passed as an option to #translate and #localize.

If no locale is passed, I18n.locale is used:

I18n.locale = :de
I18n.t :foo

Explicitly passing a locale:

I18n.t :foo, :locale => :de
I18n.l, :locale => :de

The I18n.locale defaults to I18n.default_locale which defaults to :en. The default locale can be set like this:

I18n.default_locale = :de

5 How to Store your Custom Translations

The Simple backend shipped with Active Support allows you to store translations in both plain Ruby and YAML format. 2

For example a Ruby Hash providing translations can look like this:

  :pt => {
    :foo => {
      :bar => "baz"

The equivalent YAML file would look like this:

    bar: baz

As you see, in both cases the top level key is the locale. :foo is a namespace key and :bar is the key for the translation “baz”.

Here is a “real” example from the Active Support en.yml translations YAML file:

      default: "%Y-%m-%d"
      short: "%b %d"
      long: "%B %d, %Y"

So, all of the following equivalent lookups will return the :short date format "%B %d":

I18n.t 'date.formats.short'
I18n.t 'formats.short', :scope => :date
I18n.t :short, :scope => 'date.formats'
I18n.t :short, :scope => [:date, :formats]

Generally we recommend using YAML as a format for storing translations. There are cases, though, where you want to store Ruby lambdas as part of your locale data, e.g. for special date formats.

5.1 Translations for Active Record Models

You can use the methods Model.human_name and Model.human_attribute_name(attribute) to transparently look up translations for your model and attribute names.

For example when you add the following translations:

      user: Dude
        login: "Handle"
      # will translate User attribute "login" as "Handle"

Then User.human_name will return “Dude” and User.human_attribute_name("login") will return “Handle”.

5.1.1 Error Message Scopes

Active Record validation error messages can also be translated easily. Active Record gives you a couple of namespaces where you can place your message translations in order to provide different messages and translation for certain models, attributes, and/or validations. It also transparently takes single table inheritance into account.

This gives you quite powerful means to flexibly adjust your messages to your application’s needs.

Consider a User model with a validation for the name attribute like this:

class User < ActiveRecord::Base
  validates :name, :presence => true

The key for the error message in this case is :blank. Active Record will look up this key in the namespaces:


Thus, in our example it will try the following keys in this order and return the first result:

When your models are additionally using inheritance then the messages are looked up in the inheritance chain.

For example, you might have an Admin model inheriting from User:

class Admin < User
  validates :name, :presence => true

Then Active Record will look for messages in this order:

This way you can provide special translations for various error messages at different points in your models inheritance chain and in the attributes, models, or default scopes.

5.1.2 Error Message Interpolation

The translated model name, translated attribute name, and value are always available for interpolation.

So, for example, instead of the default error message "can not be blank" you could use the attribute name like this : "Please fill in your %{attribute}".

  • count, where available, can be used for pluralization if present:
validation with option message interpolation
confirmation :confirmation -
acceptance :accepted -
presence :blank -
length :within, :in :too_short count
length :within, :in :too_long count
length :is :wrong_length count
length :minimum :too_short count
length :maximum :too_long count
uniqueness :taken -
format :invalid -
inclusion :inclusion -
exclusion :exclusion -
associated :invalid -
numericality :not_a_number -
numericality :greater_than :greater_than count
numericality :greater_than_or_equal_to :greater_than_or_equal_to count
numericality :equal_to :equal_to count
numericality :less_than :less_than count
numericality :less_than_or_equal_to :less_than_or_equal_to count
numericality :odd :odd -
numericality :even :even -
5.1.3 Translations for the Active Record error_messages_for Helper

If you are using the Active Record error_messages_for helper, you will want to add translations for it.

Rails ships with the following translations:

          one:   "1 error prohibited this %{model} from being saved"
          other: "%{count} errors prohibited this %{model} from being saved"
        body:    "There were problems with the following fields:"

5.2 Overview of Other Built-In Methods that Provide I18n Support

Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers. Here’s a brief overview.

5.2.1 Action View Helper Methods
  • distance_of_time_in_words translates and pluralizes its result and interpolates the number of seconds, minutes, hours, and so on. See datetime.distance_in_words translations.
  • datetime_select and select_month use translated month names for populating the resulting select tag. See date.month_names for translations. datetime_select also looks up the order option from date.order (unless you pass the option explicitely). All date selection helpers translate the prompt using the translations in the datetime.prompts scope if applicable.
  • The number_to_currency, number_with_precision, number_to_percentage, number_with_delimiter, and number_to_human_size helpers use the number format settings located in the number scope.
5.2.2 Active Record Methods
  • human_name and human_attribute_name use translations for model names and attribute names if available in the activerecord.models scope. They also support translations for inherited class names (e.g. for use with STI) as explained above in “Error message scopes”.
  • ActiveRecord::Errors#generate_message (which is used by Active Record validations but may also be used manually) uses human_name and human_attribute_name (see above). It also translates the error message and supports translations for inherited class names as explained above in “Error message scopes”.

* ActiveRecord::Errors#full_messages prepends the attribute name to the error message using a separator that will be looked up from activerecord.errors.format.separator (and which defaults to ' ').

5.2.3 Active Support Methods
  • Array#to_sentence uses format settings as given in the support.array scope.

6 Customize your I18n Setup

6.1 Using Different Backends

For several reasons the Simple backend shipped with Active Support only does the “simplest thing that could possibly work” for Ruby on Rails 3 … which means that it is only guaranteed to work for English and, as a side effect, languages that are very similar to English. Also, the simple backend is only capable of reading translations but can not dynamically store them to any format.

That does not mean you’re stuck with these limitations, though. The Ruby I18n gem makes it very easy to exchange the Simple backend implementation with something else that fits better for your needs. E.g. you could exchange it with Globalize’s Static backend:

I18n.backend =

6.2 Using Different Exception Handlers

The I18n API defines the following exceptions that will be raised by backends when the corresponding unexpected conditions occur:

MissingTranslationData       # no translation was found for the requested key
InvalidLocale                # the locale set to I18n.locale is invalid (e.g. nil)
InvalidPluralizationData     # a count option was passed but the translation data is not suitable for pluralization
MissingInterpolationArgument # the translation expects an interpolation argument that has not been passed
ReservedInterpolationKey     # the translation contains a reserved interpolation variable name (i.e. one of: scope, default)
UnknownFileType              # the backend does not know how to handle a file type that was added to I18n.load_path

The I18n API will catch all of these exceptions when they are thrown in the backend and pass them to the default_exception_handler method. This method will re-raise all exceptions except for MissingTranslationData exceptions. When a MissingTranslationData exception has been caught, it will return the exception’s error message string containing the missing key/scope.

The reason for this is that during development you’d usually want your views to still render even though a translation is missing.

In other contexts you might want to change this behaviour, though. E.g. the default exception handling does not allow to catch missing translations during automated tests easily. For this purpose a different exception handler can be specified. The specified exception handler must be a method on the I18n module:

module I18n
  def self.just_raise_that_exception(*args)
    raise args.first

I18n.exception_handler = :just_raise_that_exception

This would re-raise all caught exceptions including MissingTranslationData.

Another example where the default behaviour is less desirable is the Rails TranslationHelper which provides the method #t (as well as #translate). When a MissingTranslationData exception occurs in this context, the helper wraps the message into a span with the CSS class translation_missing.

To do so, the helper forces I18n#translate to raise exceptions no matter what exception handler is defined by setting the :raise option:

I18n.t :foo, :raise => true # always re-raises exceptions from the backend

7 Conclusion

At this point you should have a good overview about how I18n support in Ruby on Rails works and are ready to start translating your project.

If you find anything missing or wrong in this guide, please file a ticket on our issue tracker. If you want to discuss certain portions or have questions, please sign up to our mailing list.

8 Contributing to Rails I18n

I18n support in Ruby on Rails was introduced in the release 2.2 and is still evolving. The project follows the good Ruby on Rails development tradition of evolving solutions in plugins and real applications first, and only then cherry-picking the best-of-bread of most widely useful features for inclusion in the core.

Thus we encourage everybody to experiment with new ideas and features in plugins or other libraries and make them available to the community. (Don’t forget to announce your work on our mailing list!)

If you find your own locale (language) missing from our example translations data repository for Ruby on Rails, please fork the repository, add your data and send a pull request.

9 Resources

10 Authors

If you found this guide useful, please consider recommending its authors on workingwithrails.

11 Footnotes

1 Or, to quote Wikipedia: “Internationalization is the process of designing a software application so that it can be adapted to various languages and regions without engineering changes. Localization is the process of adapting software for a specific region or language by adding locale-specific components and translating text.”

2 Other backends might allow or require to use other formats, e.g. a GetText backend might allow to read GetText files.

3 One of these reasons is that we don’t want to imply any unnecessary load for applications that do not need any I18n capabilities, so we need to keep the I18n library as simple as possible for English. Another reason is that it is virtually impossible to implement a one-fits-all solution for all problems related to I18n for all existing languages. So a solution that allows us to exchange the entire implementation easily is appropriate anyway. This also makes it much easier to experiment with custom features and extensions.


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