Guest users should be able to change the board's displayed language (for their session). Ideally, this change would involve a dropdown box containing a list of installed language packs in the forum header, which would append the language to the url (i.e. "?lang=xx"). This way users who do not know the board's default language and who are not familiar with phpBB's default interface (or even with the interface of a style that is different from (pro/sub)silver) are able to change the language to their own (if available).
This shouldn't be too big of a change, simply check the URL to see if $_GET['lang'] is present and whether or not the chosen language is available for use, and if so, override the board's default language. Of course this would need to also only change the language for the current user and only for the current session.
Perhaps adding a column on the sessions table?