how do I - Initial user language selection for all existing users

[dacho]

Is there a query to choose for all existing users of their language?

[vBET]

User default language is stored in table user in column vbet_def_lang. So you can check it just by query to user table. For example this will tell you popularity of languages (not tested):

SELECT vbet_def_lang, count(*)
FROM user
GROUP BY vbet_def_lang