Invalid language codes in languages codeset

We are of the same opinion. This is a case where the archetype author has
to act. There are two specific cases:

- If the archetype to be included does not have any of the languages of the
container archetype, then warn the user that he has to add those
translations and do not allow solving the slot. Maybe it could be enough to
to force to have at least a translation to the original language of the
container archetype.

- If the archetype to be included only has a regional variation of the
language (for example, adding an 'en-GB' to an 'en' archetype). Can we
safely assume an automatic conversion from en-GB to en? An automatic
conversion from en to en-GB or from en-GB to en-US wouldn't be safe in any
case.

David