I think you can do something like:
session.language = session.language or T.accepted_language
auth.settings.extra_field['wiki_page']=[Field('language',default=session.language)]
auth.define_tables(....)
auth.wiki(render=False)
db.wiki_page._common_filters.append(lambda q:db.wiki_page.language==session.language)
I am not sure this belongs in the wiki because I am not convince there is one way to do it that suits everybody.
Please check it. Perhaps you can help us make a web2pyslice.
Massimo