[_ck_]

You can set a cookie and make it semi-permanent.

Because there is also a filter in that function, you do not need to hack the core.

$locale = apply_filters('locale', $locale);

You would make a mini-plugin something like this

add_filter('locale', 'language_switcher');

function language_switcher($locale) {

if (!empty($_REQUEST['lang'])) {$locale=substr(strip_tags(stripslashes($_REQUEST['lang'])),0,30);}

return $locale;

}

You still have to write the code to make some kind of pulldown/menu to select the language and set the cookie.

There is a language switcher for wordpress that could be ported to bbPress, I think there was some talk about it around here and someone else made their own too.

Oh here it is:

https://bbpress.org/plugins/topic/user-languages