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Translation engine for Rails applications


>= 0.7, < 2
>= 2.7
 Project Readme

ZLocalize - A translation engine for Rails 6+ applications

ZLocalize provides string translation through YAML-defined dictionaries.

What it does:

  • It uses strings in the source code as keys for lookups with interpolation of values.
  • It features a harvester rake task which scans the application source code (controllers, models, helpers and views) and to extract all the strings used in calls to translation methods.
  • The harvester also manages the translation entries, e.g. each time it runs, it will update the translation dictionaries without destroying existing entries or creating duplicate entries.
  • Translation dictionaries are stored as YAML files, containing metadata (such as unique entry ID's and a list of references where an entry is used, much like gettext .po files).
  • Provides a simple way to manage used-generated content in multiple languages by including modules an ActiveRecord.
  • Provides conversion of locale-formatted decimal values assigned to ActiveRecord attributes by users (through forms, console, etc.).

What it does not do:

  • Localization of date/time or other regional-dependent data (except for input of decimal values). For localization, you must use I18n#localize.
  • Provide an interface to edit and manage translations. You have to edit the YAML files by hand.

Within a Rails application, ZLocalize does not replace the I18n backend. Its operation is (almost) entirely independent from I18n. The only tie to I18n is the internal use of the I18n#locale and I18n#default_locale methods to get/set its own active and default locales. Setting the active locale through ZLocalize will set I18n's locale and vice-versa.

Other than that, ZLocalize keeps its translation data isolated from the I18n translation data.

The reason for this architecture is that the Rails framework itself contains a finite set of strings, and as such I18n is already doing a fine job with translation and localization of the framework internal strings and helpers, using a hierarchical data store with Ruby symbols as keys.

One of the main reasons ZLocalize was created was to avoid having to deal with symbols as keys while developing. It is much easier and straightforward to start coding your application using a base locale, and then run the harvester to collect all the strings used in the translation calls throughout the source code.


  • Ruby 2.4 or later
  • Rails 6.0 or later

If you really must use this gem with a previous version of Ruby (< 2.0) or Rails (< 6.0), install ZLocalize 4.2.3.


Add to your Gemfile:

gem 'zlocalize'

and run +bundle install+

Basic configuration

  1. Create an initializer:

     bin/rails generate zlocalize:initializer

    This will create config/initializers/zlocalize.rb with some default configuration values.

    You can configure I18n and ZLocalize in each their own initializer files, or you can configure both in the same initializer.

    First, set the default locale:

     I18n.default_locale = :en

    Calling I18n.default_locale and I18n.locale is the same as calling ZLocalize.default_locale and ZLocalize.locale. The two are bound together.

  2. Edit the ZLocalize.config.locales structure, by specifying a Hash for each locale you want to support. Each locale definition is a key/value pair. For example, the French locale definition could be:

     ZLocalize.config.locales = {
       :fr => {
         :plural_select    => -> (n) { n <= 0 ? 0 : (n > 1 ? 2 : 1) },
         :translations     => File.join(Rails.root,'config/translations/fr.strings.yml'),
         :titleize         => -> (s) { s.capitalize.to_s },
         :convert_float    => -> (s) { s.to_s.gsub(' ','').gsub(',','.') }
    • :plural_select is a proc evaluating the value of n for translation of plural strings (in calls to ZLocalize#pluralize or n_). It returns the index of the Array element to use as the translation for plural expressions.

    • :translations is an Array (or a single entry) of file names where the translations are stored.

    • :titleize is a proc to be used instead of ActiveSupport's titleize helper. Call ZLocalize.titleize instead of the builtin titleize (helper or ActiveSupport::Inflector method) in your application.

    • :convert_float is a proc to be used to convert ActiveRecord attributes containing decimal values. For example, in French, if the String "1 234,23" is assigned to a AR instance, it will be converted to "1234.23" so that Rails can then correctly convert it to BigDecimal internally.

    Note that you can also call ZLocalize#convert_float anywhere in your application.

  3. Set the other configuration values:

  • define_gettext_methods : define _() and n_() (on Object class). Defaults to true. If you do not define the gettext methods, you will need to call ZLocalize.translate and ZLocalize.pluralize in your application.

  • return_source_on_missing: Hash with a key/value pair for each of your application environments. This indicates if missing translations in a given locale should raise a ZLocalize::MissingTranslationDataError exception. Defaults to { :development => true, :test => false, :production => false, :staging => false }

  • harvest_paths : Array of path patterns (same as Dir.glob) relative to Rails.root that the ZLocalizer Harvester (see Harvester section below) will scan to collect all calls to _(), n_(), ZLocalize.translate and ZLocalize.pluralize. Defaults to ["app/channels/**/*.rb", "app/controllers/**/*.rb", "app/helpers/**/*.rb", "app/models/**/*.rb", "app/views/**/*.erb", "app/mailers/**/*.rb", "app/jobs/**/*.rb", "lib/**/*.rb" ]

In your application (Controllers, Helpers, Views, Models, etc.)

The idea is to simply start coding your application without thinking too much about how to translate it. The only thing to worry about is wrapping any String you will eventually want translated in a call to _() or n_() (or ZLocalize.translate and ZLocalize.pluralize). Make sure you use parenthesis for the parameter list to those methods (see Harvester section below for an explanation).

For example, in a view:

<%= _("Dear user") %>
<%= n_(["No messages","One message", "{{count}} messages"], @user.messages.count) %>

Or in a controller:

def create
  @post =[:post])
    flash[:notice] = _("Your post has been added.")
    redirect_to [@post] and return
    flash[:error] = _("There was an error adding your post")

There are 2 methods to translate content:

  • ZLocalize.translate(key, options = {}) (and its gettext-style alias _(key, options = {})). This method looks up the String key with the following options:

    • :locale : Lookup key in this locale. Defaults to I18n.current_locale.
    • :default : Return this String if key is not found.
    • :return_source_on_missing : Override the global ZLocalize.config.return_source_on_missing value.

    Any other option key passed is meant to be an interpolated named value. See Interpolation section below.

  • ZLocalize.pluralize(key, count, options = {}) (and its gettext-style alias n_(key, count, options = {})). This methods looks up the Array key (which is an Array of String), and computes the index of the String to return based on the value of count. The index is computed using the :plural_select Proc from ZLocalize.config (see Basic Configuration section above).

    The options are the same as the translate method. Also, the count parameter is also automatically treated as a value to be interpolated (when {{count}} token is present in translation string).


ZLocalize supports interpolation of values in source strings. Simply enclose the values in double curly braces ({{ and }}), and pass the actual values in a Hash as the 2nd parameter to ZLocalize.translate and ZLocalize.pluralize. For example:

<%= _("Hello, {{username}}! How are you {{moment}}?",
        :username =>,
        :moment => > 18 ? _("tonight") : _("today") ) %>

If you need to output actual double curly braces, simply double-escape them:

<%= _("Hello, \\{{username}}") %>

The above will return "Hello, {{username}}" without interpolation of {{username}}.

Namespacing Source Strings

It is possible to namespace (to give a scope to) a source string by simply prefixing it with a name that ends with :: . The namespace is used when looking up the key in the current locale, but the prefix will always be stripped in any output (even in the default locale).

Let's say your default locale is 'en':

<%= _('btn::Edit') %>

The above returns "Edit" if the current locale is the base locale (en in this case), and will lookup "btn::Edit" (e.g. including namespace) for all other locales and return the corresponding translation. The translation may include a scope, but it will be removed too.

The idea behind such scopes is that some languages have different spelling or even words for a given word in another locale. For example, "Update" in English can be used both as a noun and a verb. But it is not always the case in other languages. By adding scopes such as "btn::Update" or "title::Update", we infer context to the string, so other languages would translate differently depending on the context (scope). And in the base locale, only the part after :: would be shown (the scope would be stripped).

Should you need to output the scope delimiter '::' as is, escape it with a backslash:

<%= _("\\::HomePage") %>

The above will lookup "\\::HomePage", but will not treat it as a namespace prefix and will return the string with only the backslash stripped (i.e. "").


Once you're ready to translate any work you've done:

bin/rake zlocalize:harvest output=config/translations/fr.strings.yml

The harvester will scan your application (models, controllers, helpers, views and any other path you might add). This will create a YAML file containing a list of entries corresponding to all the strings used in the calls to _() and n_() in your source code.

However, you have complete control over the location of your ZLocalize translation files and can store them in any subdirectory of your Rails application. See Basic Configuration section above.

Use rake -D zlocalize:harvest for a list of options.


The parser used by the harvester module does have some limitations:

  1. You must use parenthesis in the calls to _() and n_() (or ZLocalize.translate and ZLocalize.pluralize). While the Ruby language allows to omit parenthesis for method parameters, the Harvester parser does require them. The rule is simple: the parameter list to a translation method (and the parameters of any nested calls) must use parenthesis.

    This will not be harvested:

     _ "Hello"
     _ "Hello, {{name}}", :name => get_user_name @user

    But this will:

     _("Hello, {{name}}", :name => get_user_name(@user))
  2. Translation calls inside interpolated double-quoted strings are not supported.

    While you can do something like:

     "This is #{_('embedded')} in a string"

    The Harvester parser will not detect the call. Avoid doing so.

Translation file format

Entries in the YAML file have the following format:

  id: 1
  plural: false
  ignore: false
    - /app/views/users/show.html.erb:12
  source: "Dear user"
  translation: "Cher utilisateur"

  id: 2
  plural: true
  ignore: false
    - /app/views/users/show.html.erb:21
    - "No messages"
    - "One message"
    - "{{count}} messages"
    - "Aucun message"
    - "Un message"
    - "{{count}} messages"

Note that for plural entries, the source is the Array of strings passed to n_() (or ZLocalize.pluralize() ) in the source file. It can have any number of elements, as required by your base locale. The translation is also an Array, and it too can have any number of elements as required by the target locale. It is up to the :plural_selector proc to compute the correct index to use in the translation array.

Updating translations when the application changes

Simply run rake zlocalize:harvest again, specifying as output the file that already contains for the target language.

The Harvester will scan the source code and add/remove/modify the references to all strings already present (if a string is not used anymore, it will be removed only if you also add the purge=true parameter). It will of course also add any missing string to the existing translation file.

Translation of user-generated content (ActiveRecord)

Any ActiveRecord model can be made to support multiple languages for its attributes. ZLocalize provides 2 mechanisms to achieve this:

1. Attached Translations

This method stores the translation of values in a separate model (judiciously) called Translation.

Any model can have multiple translated values in multiple locales attached to it. These values essentially become attributes of the model.

To use attached translations for a given model, declare has_translations for that model. For example:

class Page < ActiveRecord::Base
  validates :title, translation: { required_locales: [:fr, :en] }
  # `required_locales` can be a Symbol, in which case it refers to a method on the instance (no parameters)
  # that will be called when the validation is performed.
  # It can also be a Proc or lambda to be called, with (record, attribute, value) as parameters
  # validates :title, translation: { required_locales: :get_required_locales }
  # validates :title, translation: { required_locales: -> (record, attribute, value) { [:es, :fr] } }
  # return an Array of locale identifiers
  def get_required_locales
    [ :de, :en ]

From then on, any instance of Page will have the following methods:

  • #translate(attr_name,locale = nil)
  • #add_translation(attr_name,locale,value)
  • #insert_translations(locales = {})

So, a typical use with our example would be:

In the controller:

def create
  @page =[:page])
  @page.add_translation('title',:fr,'Mon titre')

And in a view (where the current locale is +:fr+):

<h3><%= @page.translate(title) %></h3>   # => "Mon titre"

If you have some kind of administration interface, you can also mass-assign the translations for a number of attributes and locales with the insert_translations method.

#insert_translations accepts a Hash containing the translations for multiple columns and locales. The Hash must have the locales as keys and its value is another Hash of name-value pairs. As in:

  { 'en' => { 'title'    => "What's new this week",
              'synopsis' => "Learn what has happened this week"},
    'fr' => { 'title'    => "Quoi de neuf cette semaine",
              'synopsis' => "Tout sur ce qui s'est passé cette semaine" }

Alternatively, you can use ActiveRecord's builtin accepts_nested_attributes_for mechanism to mass-assign translations:

class Article < ActiveRecord::Base
  accepts_nested_attributes_for :translations, :allow_destroy => true

And use form.fields_for :translations inside a view to have the translations assigned directly...

The Translation model is loaded by ZLocalize itself, whenever a model declares has_translations. The only thing missing is the translations table, which you can generate with:

rails generate zlocalize:translations_migration

And run rake db:migrate.

Attached translations are very flexible, because the values are not stored in the model itself (as opposed to translated columns, explained in the next section below). This allows to easily add new locales with minimal changes to the models that use them. All this at the cost of having an association with the Translation model.

Validation of attached translations

ZLocalize also adds a Validator to ActiveRecord::Base, to check the presence of attached validations:

class Page < ActiveRecord::Base

  validates_translation_of :content, :required_locales => [:fr, :en]
  # or #
  validates :content, :translation => { :required_locales => [:fr, :en] }
  # or #
  validates :content, :translation => { :required_locales => :get_required_locales }

  def get_required_locales
    [:es, :en]


This will generate an error message for each missing translation:

p = @page.create

p.errors[:content]    # => ["content is missing its translation in fr, en"]

The required_locales option can also be a Proc/lambda:

validates_translation_of [:title, :content], :required_locales => -> (record,attribute,value) {
                                                if attribute == 'content'
                                                end }

The other standard validation options, such as :message, :on, :if and :unless are also supported. :message defaults to :missing_translations and this value should be added to the I18n translations for Rails, with the other ActiveRecord validation messages:

      messages: &errors_messages
        inclusion: "n'est pas inclus(e) dans la liste"
        exclusion: "n'est pas disponible"
        missing_translations: "doit ĂȘtre traduit en %{locales}"

and the %{locales} token will be interpolated to the actual missing locales.

2. Translated columns

This second attribute translation mechanism works a bit differently. It is basically a wrapper around column names, allowing to use different attribute values for each locale.

First, you need to create the columns in the model table. You need one column for each attribute and locale pair. For example, a title attribute with values in French and English would require 2 columns:

create_table 'pages' do |t|
  t.string :title_en
  t.string :title_fr

In your model:

class Page < ActiveRecord::Base
  translates_columns [:title]

Then whenever you want to access the value of +title+ in the current locale:

<%= @page.title %>

The wrapper method can take a Hash of options:

  • :locale to force a given locale
  • :fetch_default (true|false) to retrieve the the value of the attribute in the default_locale. Defaults to true

For example:

<%= @page.title(:locale => :fr, :fetch_default => false) %>

This will automatically call the wrapper method to read the value of the column in the :fr locale (the value of title_fr). If title_fr is nil, then the value in the default_locale will not be fetched and nil will be returned. If fetch_default is true, then the value of title_en (given that the default_locale is :en) would be fetched.

Translated columns are more efficient in terms of fetching translated content, because they are always part of the underlying table columns. If you know in advance the exact locales you are going support in your application, and that only 2-3 locales will be present, then having multiple columns for each attribute is probably the way to go. If you have many locales and/or the attributes to be translated are large (text columns and such), then maybe Attached Translations would be a better strategy.

Localized Decimal Attributes

ZLocalize provides a way to declare some ActiveRecord attributes as being localized decimal values. This ensures the assignment of decimal values from their string-based representation will not fail. For example, in French, the decimal separator is a period ('.') and the thousands separator is a space. Assigning the string "1 234,56" to a decimal attribute will cause the value to become 1.0 (the BigDecimal class will parse the string, but stop at the space).

To enable correct decimal attribute assignment for different locales, declare the attributes as such:

class Account < ActiveRecord::Base
  localize_decimal_attributes [:balance, :variation]

Then, you can safely assign a string representing a decimal value in the current locale:

Zlocalize.locale = :fr
@account.balance = "8 765,43"
=> 8765.43

Conversion from a locale-specific decimal value to BigDecimal is done by the :convert_float Proc declared in ZLocalize.locales (in the initializer). For example, in English:

  ZLocalize.config.locales = {
    :en => {
       :plural_select     => -> (n) { n <= 0 ? 0 : (n > 1 ? 2 : 1) },
       :convert_float     => -> (s) { s.to_s.gsub(',','') }


ZLocalize is released under the MIT license.


Source code, documentation, bug reports, feature requests or anything else is at


This plugin is based on original work by Thomas Fuchs (A Rails 1.X plugin named Localization), but it has been extended in many ways to make it answer our needs.

Many thanks to Stephane Volet ( for his contributions and ideas to this gem.