As per Summary. Probably simple replace of linguas_${u} to l10n_${u} will do the trick, but it needs to be checked
It is not so easy. For the time being, l10n.eclass needs to keep the functions for manipulation of LINGUAS (even if the name of the eclass is unfortunate). Not sure if we will need a similar eclass for L10N. In any case, that should be handled in a new bug then.