One example of such feature you can find in the /acs-lang/www/index page. If, for instance, you go to
https://openacs.org/acs-lang/, there you will have the possibility to switch the preferred locale for you on this website.
Of course your application should have been written with localization in mind, that is, have every UI text as a localized message key (with a proper translation for the languages you want to support).
Hope this helps
Antonio
