Rack locale setter middleware

sinatra rack ruby i18n

For a project I am currently working on I am using the nice i18n library r18n which provides i18n for Rails, Sinatra and most general Ruby projects. It also is reasonably ORM agnostic and so I am using it quite easily with DataMapper.

I have decided to use a subdomain to specify the locale rather than having it be part of the path. So instead of this:


I have this:


I find this makes it easier by being able to specify relative paths everywhere that don’t need to have the locale played with. Switching the locale for the site is also just a matter of changing the domain.

That said, r18n expects the locale to be placed in the ‘params’ array for Sinatra and I thought a simple Rack middleware would do this well. So here it is:

module Rack
  class LocaleSetter
    def initialize(app)
        @app = app

    def call(env)
        req = Rack::Request.new(env)
        if m = req.host.match(/^(?:www\.)?([a-z]{2})\./)
        locale = m[1]
        locale = 'en'
        req.params['locale'] ||= locale
        @app.call env

Due to many peoples’ habit of putting ‘www’ at the start of the domain, this should account for en.example.com and www.en.example.com. It also allows for the locale to be specified in the query string. So en.example.com/foo?locale=th will set the locale to ‘th’.

There are probably others out there but due to the simplicity of Rack based apps it is very easy to write, and adjust, your own.