rack-i8n_best_langs: guess best language for content served over Rack

rack-i18n_best_langs is a Rack middleware component that takes care of understanding what are the best languages for a site visitor.

If you manage a site that has content many languages and also localized URLs, you will find rack-i18n_best_langs very useful, especially when used in conjunction with rack-i18n_routes.

Differently from other similar Rack middleware components, rack-i18n_best_langs returns a list of languages in order of guessed importance, not a single language. Also, it does not require Rails or the i18n gem.


Language discovery is done using three clues:

  • the presences of language tags in paths (e.g. /service/warranty/ita),
  • the content of the HTTP Accept-Language header,
  • the content of the rack.i18n_best_langs cookie when set.

All these clues are taken into account and evaluated against the list of languages available and their preferred order. It is possible to configure which of these clues is the most important.

An additional clue is available when AliasMapping (part of rack-i18n_routes) is used as the mapping function: the language in which the path is written. For example, /articles/the-victory add preference of English, /artículos/la-victoria for Spanish and /articles/la-victoire for French.

How it works

Each request is analysed in search for possible clues that can expose what are the best languages to use in the content served in response. Each clue contributes part of the final score of each language; the score is amplified by the weight associated to the clue.

For example, if a person requests the URL /article/vittoria/ita from a browser in a German internet point (thus sending Accept-Language: de-DE), their request would lead to the following scores:

  • ita: 315 = 0 (not in header) + 15 (partially language of URL) + 300 (path ends in /ita);
  • eng: 15 = 0 (not in header) + 15 (partially language of URL) + 0 (path does not end in /eng);
  • ger: 3 = 3 (in header) + 0 (not language of URL) + 0 (path does not end in /ger).

The downstream application will find the guessed languages in the rack.i18n_best_langs env variable, in order of importance.

env['rack.i18n_best_langs'] # => [ 'ita', 'eng', 'ger' ]

At this point, it is up to the application to choose what to do, either change the locale using the i18n gem, set up its own locale management system or just keep track of this information.


rack-i18n_best_langs works like any other Rack middleware component.

# in your server.ru rackup file
require 'rack/i18n_best_langs'

FAVORITE_LANGUAGES = %w(eng spa deu fra)

use Rack::I18nBestLangs, FAVORITE_LANGUAGES
run MyApp

In your application you will find the list of languages that should be used to serve the content, arranged from the most favorite to the least in the rack.i18n_best_langs Rack variable. It is then up to downstream application to use this information in the best way.

See the guessed languages

This small application

# in your server.ru rackup file
require 'rack/i18n_best_langs'

FAVORITE_LANGUAGES = %w(eng spa deu)

use Rack::I18nBestLangs, FAVORITE_LANGUAGES

app = Proc.new do |env|
    langs = env['rack.i18n_best_langs']
    [200, {"Content-Type" => "text/plain"}, [langs.inspect] ]

run app

will produce the following results for these URLs.

# /foo =>
#    [#<LocaleCode 'eng'>, #<LocaleCode 'spa'>, #<LocaleCode 'deu'>]

# /foo/spa =>
#    [#<LocaleCode 'spa'>, #<LocaleCode 'eng'>, #<LocaleCode 'deu'>]

# /foo (with Accept-Language = it-IT, es-ES, fr-FR) =>
#    [#<LocaleCode 'spa'>, #<LocaleCode 'eng'>, #<LocaleCode 'deu'>]

# /foo/deu (with Accept-Language = it-IT, es-ES) =>
#    [#<LocaleCode 'deu'>, #<LocaleCode 'spa'>, #<LocaleCode 'eng'>]

# /foo (with cookie set to 'deu') =>
#    [#<LocaleCode 'deu'>, #<LocaleCode 'eng'>, #<LocaleCode 'spa'>]

# /foo/spa (with cookie set to 'deu') =>
#    [#<LocaleCode 'deu'>, #<LocaleCode 'spa'>, #<LocaleCode 'eng'>]

Changing the clues’ weights

You can tune the weights of the clues to set which clue is the most important.

The default order of importance and weights are

  • language set in cookie (:cookie): 3
  • language present in tag (:path): 2
  • language is in Accept-Language header (:header): 1

You can change these weight with the :weights option.

FAVORITE_LANGUAGES = %w(eng spa deu)
WEIGHTS = { :path => 3, :header => 2, :cookie = 1 }

use Rack::I18nBestLangs, FAVORITE_LANGUAGES, :weights => WEIGHTS

To disable the use of any of the clues, set its weight to zero.

Using AliasMapping

If you want to use the content of the URI path as an additional clue to guess the best languages, use an AliasMapping function as path mapping function.

# in your server.ru rackup file
require 'rack/i18n_best_langs'
require 'rack/i18n_routes/alias_mapping'

FAVORITE_LANGUAGES = %w(eng spa deu fra)

aliases = {
    'articles' => {
        'fra' => 'articles',
        'spa' => ['artículos', 'articulos']

        :children => {
            'the-victory' => {
                'fra' => 'la-victoire',
                'spa' => 'la-victoria'
            'the-block' => {
                'fra' => 'le-bloc',
                'spa' => 'el-bloque'
MAPPING = Rack::I18nRoutes::AliasMapping.new(paths, :default => 'eng')

use Rack::I18nBestLangs, FAVORITE_LANGUAGES, :path_mapping_fn => MAPPING
run MyApp


No requirements outside Ruby >= 1.8.7 and Rack.


gem install rack-i18n_best_langs



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This is free software released into the public domain (CC0 license).

See the COPYING file or http://creativecommons.org/publicdomain/zero/1.0/ for more details.