Various places use an empty locale or string to indicate different
meanings and/or variants of "this is not a real locale but I didn't know
what to use else" or abuse LANGUAGE_DONTKNOW in one or the other way. In
preparation of changing that awkward situation now offer the ISO 639
codes for multiple languages [mul] and language undetermined [und],
mapping to LANGUAGE_MULTIPLE and LANGUAGE_UNDETERMINED.

Change-Id: I687de23ffc00a9a056f2837b024e0a62658e3df2