Redmine

I run documentation in wiki in 3 languages now (English, Russian, Spanish. When anonymous user comes from Spanish site I want to change language of Redmine to Spanish, the same for Russian. I also run forums on Redmine for different languages, as well I want user to switch language without being registered.

I suggest I can write some hack that will change language by url parameter and store this value in cookie
http://localhost/redmine/wiki/project?lang=es

Can anybody give me some directions where to start from?

P.S.: btw It would be nice to have pulldown menu for languages the same as for projects in top right corner