(Shameless self-promotion) You can also use my translate_routes plugin:
which not only adds a locale parameter and prefix:
But also allows you to translate the static fragments of your routes,
improving your SEO performance. Your current xxxxx_path and xxxxx_url
helpers will be transparently converted to the current request's
locale, so you just need to add a .yml translations file and one line
to your config/routes.rb.
Hope it helps!