When doing QA on core sch, we figured there is an important bug to fix in
XLIFF 2 Core.
While xml:lang on target is constrained to match trgLang, this Constraint
is missing on source.
This basically means that multisource language XLIFFs could be created and
we couldn't declare XLIFF files with xml:lang on source different from
srcLang as invalid.
I believe this omission should be fixed both in prose and the core