Хак для XOOPS v.1.0 RC3.04 добавляет Блок для выбора языка.
В работе можно посмотреть - здесь:
http://www.artistic.ro/xoops/h_select_language.htm
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This hack is intended to let the user select his own language on the fly.
Although the XOOPS engine allows for selecting a default language, it doesn't
allow for selecting the language "on the fly". This hack consists of a block
which displays a list of flags, or a drop down combo (depending on the settings)
with the current available languages. When a user selects a language, the
information is stored in a cookie, for the current domain.

NOTE: The language changing is only for the interface! This means the
information in the database (news/forums/etc..) will still be displayed in the
language that the author used!

I think this should have been inside XOOPS for a long time now, but maybe the
developers will have the kindness of including it in by "default"