Assigning languages

How do I change someone else’s Personal Language? (Preferences >Personal Language.) And is this different from First Language?

Personal Language is the language they view the interface in Gibbon using, which can only be set to one of the active languages installed in your system, whereas the First, Second, etc Language fields are for indicating which languages they speak. These can be similar but not always the same. Personal Language is set by the user, there isn’t a way to set it for them, aside from changing the System Language which changes the default interface language for all users (or perhaps creating a custom import if you need to import that kind of data).

That answers my question. On a multi-lingual site I (the administrator) cannot set individual’s personal language. Thanks