Rack::LocaleSelector is designed to provided locale detection settings through the middleware stack with I18n.
The behaviour is quite similar to google's locale settings: It tries first to fetch locale value from URI, cookie, HTTP_LANGUAGE_HEADERS or fallback to I18n default's value.
Rack::LocaleSelector sets the I18n.locale to the matching value or redirects the user to the proper locale in URI, here is how redirection work:
http://example.com (with cookie set to :fr) => http://fr.example.com http://example.com (with HTTP_ACCEPT_HEADERS set to :fr) => http://fr.example.com tricky ones: http://es.example.com (with cookie set to :fr) => http://es.example.com (sets a cookie to :es)
You can choose to not use locale as subdomain but as a value in URI (example.com/fr) by setting :use_subdomain to false
script/plugin install git://github.com/jeremyvdw/rack-locale-selector.git # config/environments/development.rb config.middleware.use "Rack::LocaleSelector" Options: :use_subdomain :default_domain :white_list # Buggy :black_list # Buggy