LT-14932 don't use invalid first variant element as language

Our heuristic of treating the first element after the x as a
private-use language code in WS identifiers like qaa-x-kal
should not be applied to ones like qaa-x-dupl0 where the
item is not a valid language code (in this case, too long).

Change-Id: I2cab254aca3d18df9ed95bb6134ded39092c203b