you set the site wide default language in site admin.
but each user can have another preferred lanuage.
the multilanguage module hasn't anything to do with how language is handled in the G2 user interface. now you may wonder why it is called multilanguage module. it's because it adds some additional multi language functionality. without the multilanguage module, you can annotate items only in a single language. with the multilanguage module, you can add descriptions to items in multiple languages.
so the multi language module doesn't handle the user interface language, it handles the sentences / annotations you wrote for certain items.
