This filter attempts to auto-detect the clients desired language. It first checks the params, then a cookie and then the HTTP_ACCEPT_LANGUAGE request header. If a language is found to match or be similar to a currently valid language, then it sets the current_language of the controller.

  class ExampleController < ApplicationController
    set_language :en
    autodetect_language_filter :except => 'monkey', :on_no_lang => :lang_not_autodetected_callback
    autodetect_language_filter :only => 'monkey', :check_cookie => 'monkey_lang', :check_accept_header => false
    ...
    def lang_not_autodetected_callback
      redirect_to somewhere
    end
  end

The args for this filter are exactly the same the arguments of before_filter with the following exceptions:

• :check_params — If false, then params will not be checked for a language. If a String, then this will value will be used as the name of the param.
• :check_cookie — If false, then the cookie will not be checked for a language. If a String, then this will value will be used as the name of the cookie.
• :check_accept_header — If false, then HTTP_ACCEPT_LANGUAGE will not be checked for a language.
• :on_set_lang — You can specify the name of a callback function to be called when the language is successfully detected and set. The param must be a Symbol or a String which is the name of the function. The callback function must accept one argument (the language) and must be instance level.
• :on_no_lang — You can specify the name of a callback function to be called when the language couldn’t be detected automatically. The param must be a Symbol or a String which is the name of the function. The callback function must be instance level.

You override the default names of the param or cookie by calling GLoc.set_config :default_param_name => ‘new_param_name‘ and GLoc.set_config :default_cookie_name => ‘new_cookie_name‘.

[Source]

    # File lib/gloc-rails.rb, line 43
43:       def autodetect_language_filter(*args)
44:         options= args.last.is_a?(Hash) ? args.last : {}
45:         x= 'Proc.new { |c| l= nil;'
46:         # :check_params
47:         unless (v= options.delete(:check_params)) == false
48:           name= v ? ":#{v}" : 'GLoc.get_config(:default_param_name)'
49:           x << "l ||= GLoc.similar_language(c.params[#{name}]);"
50:         end
51:         # :check_cookie
52:         unless (v= options.delete(:check_cookie)) == false
53:           name= v ? ":#{v}" : 'GLoc.get_config(:default_cookie_name)'
54:           x << "l ||= GLoc.similar_language(c.send(:cookies)[#{name}]);"
55:         end
56:         # :check_accept_header
57:         unless options.delete(:check_accept_header) == false
58:           x << %<
59:               unless l
60:                 a= c.request.env['HTTP_ACCEPT_LANGUAGE'].split(/,|;/) rescue nil
61:                 a.each {|x| l ||= GLoc.similar_language(x)} if a
62:               end; >
63:         end
64:         # Set language
65:         x << 'ret= true;'
66:         x << 'if l; c.set_language(l); c.headers[\'Content-Language\']= l.to_s; '
67:         if options.has_key?(:on_set_lang)
68:           x << "ret= c.#{options.delete(:on_set_lang)}(l);"
69:         end
70:         if options.has_key?(:on_no_lang)
71:           x << "else; ret= c.#{options.delete(:on_no_lang)};"
72:         end
73:         x << 'end; ret }'
74:         
75:         # Create filter
76:         block= eval x
77:         before_filter(*args, &block)
78:       end