(Shameless self-promotion) You can also use my translate_routes plugin:
http://github.com/raul/translate_routes/
which not only adds a locale parameter and prefix:
http://localhost:3000/en/articles/1/comments/1 http://localhost:3000/es/articulos/1/comentarios/1
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!