Accept Language

A tiny library for parsing the Accept-Language header from browsers (as defined in RFC 2616).

Status

Installation

Add this line to your application's Gemfile:

gem 'accept_language'

And then execute:

$ bundle

Or install it yourself as:

$ gem install accept_language

Usage

It's intended to be used in a Web server that supports some level of internationalization (i18n), but can be used anytime an Accept-Language header string is available.

Examples:

AcceptLanguage.parse('da, en-gb;q=0.8, en;q=0.7')                     # => {:da=>1.0, :"en-gb"=>0.8, :en=>0.7}
AcceptLanguage.parse('fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5')  # => {:"fr-ch"=>1.0, :fr=>0.9, :en=>0.8, :de=>0.7, :*=>0.5}

In order to help facilitate better i18n, a method is provided to return the intersection of the languages the user prefers and the languages your application supports. Two letter truncation is enabled by default and truncates both the input language tags and available language tags to 2 characters for a broadly permissive match; both en and en-GB will match any collection containing a language starting with en, including e.g. en-NZ. If this is disabled, full tag matches are required, so en-GB would only match en-GB and not en-NZ.

Examples:

AcceptLanguage.intersection('da, en-gb;q=0.8, en;q=0.7', :ja, :ro, :da)    # => :da
AcceptLanguage.intersection('da, en-gb;q=0.8, en;q=0.7', :ja, :ro)         # => nil
AcceptLanguage.intersection('fr-CH', :'fr-CA', two_letter_truncate: false) # => nil
AcceptLanguage.intersection('fr-CH', :'fr-CA', two_letter_truncate: true)  # => :'fr-CA'
AcceptLanguage.intersection('de, zh;q=0.4, fr;q=0', :fr)                   # => nil
AcceptLanguage.intersection('de, zh;q=0.4, *;q=0.5, fr;q=0', :fr)          # => nil
AcceptLanguage.intersection('de, zh;q=0.4, *;q=0.5, fr;q=0', :ar)          # => :ar

The returned capitalisation of the list of supported languages/tags is always preserved, though the return value will always be a Symbol or nil. This is important for Rails (see below) since I18n.locale = ... is case-sensitive. If you need it, you can specify option enforce_bcp47: true to enforce BCP 47 capitalisation rules for returned matches instead.

Rails integration example

In most cases, you will probably want to try a strict match followed by a permissive match, dropping to default locale if all else fails.

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  before_action :best_locale_from_request!

  def best_locale_from_request!
    I18n.locale = best_locale_from_request
  end

  def best_locale_from_request
    return I18n.default_locale unless request.headers.key?('HTTP_ACCEPT_LANGUAGE')

    # Try an exact match first, then use two letter fallback to select
    # any arbitrary first-two-letters-match choice from the collection
    # of available locales; e.g. "en-GB" requested could lead to "en-US"
    # being selected based on the "en" match. This may not be what you
    # want for things like "zh" for Chinese variants, in which case the
    # "if locale.nil?" truncate-true line should be removed.
    string = request.headers.fetch('HTTP_ACCEPT_LANGUAGE')
    locale = AcceptLanguage.intersection(string, I18n.default_locale, *I18n.available_locales, two_letter_truncate: false)
    locale = AcceptLanguage.intersection(string, I18n.default_locale, *I18n.available_locales, two_letter_truncate: true) if locale.nil?

    # If the server cannot serve any matching language,
    # it can theoretically send back a 406 (Not Acceptable) error code.
    # But, for a better user experience, this is rarely done and more
    # common way is to ignore the Accept-Language header in this case.
    return I18n.default_locale if locale.nil?

    locale
  end
end

Versioning

AcceptLanguage uses Semantic Versioning 2.0.0

License

The gem is available as open source under the terms of the MIT License.