babel/polyglossia

Currently the package loads babel in luatex and polyglossia in xetex.

This means that the in-document commands for language switching change when you change engines and makes it much harder to make compatible, cross-engine documents.

I think it would be much better if instead of testing the engine you test if polyglossia is loaded, if so use polyglossia, otherwise load and use babel. That way the user has a choice to load or not load polyglossia and the behaviour does not automatically change when changing engines,