3 options (about putting language plugin) (when i finished writing it i only then saw that u have WPML witch i didnt know was a lang plugin so maybe useless my post or u can give argument why not use my version?
------Plugins and some my www tests with language plugins
They some weeks later will be changed so check them now while im doing tests on them.
1--- Polylang http://wordpress.org/extend/plugins/polylang/
Just one plugin witch links pages to their translations and makes option to ur blog be with either http://www.domain.com/en/ or en.domain.com
So u cant use it without programming knowledge to change it to make it like u want to be hungarian different language.
This plugin makes translated topics as new page so comments are seperated.
My test examples http://tls.lv and http://talantufabrika.lv
2--- qTranslate http://wordpress.org/extend/plugins/qtranslate/
This u need to use if u will translate all languages u install in plugin other wise untranslated will show up with info its not translated. For comments for language sepperation ull need also qTranslate Separate Comments.
I would recommend u to do multisite installation and then u dont need language plugin but all installed plugins and themes u can share across domains.
Well but then about bbpress.. I think maybe thouse who use one lang will not go to other or ull need some multi bbpress modification so it works across all multisites with one login. Or some user data sharing across them.