Found when investigating fdo#86011 where 'du-nl' was accepted and
assigned an on-the-fly-ID even if the 'du' language code is not defined
in ISO 639. If a tag is not internally known we need to check it using
liblangtag and take no short cut assuming it would be alright even not
for the simplest case.

Change-Id: Ib8c1eb77b6b4a59019c4c8248b596bf53c0f4986