Fix usage of multiple varieties of the same polyglossia language.

Fix usage of multiple varieties of the same polyglossia language.

1. We must always output all (diverging) options, including
   default options; if not, default options might get overwritten.

2. Do not output options in \setotherlanguage, since we might have
   multiple "other languages" varieties from the same language (such
   as naustrian, nswissgerman). And the options are output for the
   language switches anyway.

Hence, LaTeXFeatures::getPolyglossiaLanguages() does not have to record
varieties. This was not done correctly anyway, since the map allowed
for one entry per language only.