Use polyglossia also with DVI (LuaTeX).

Use polyglossia also with DVI (LuaTeX).

Simplify the logic for language package selection and make it more consistent:

Use polyglossia with non-TeX fonts (system fonts/Unicode fonts) for all
export flavours (XeTeX, LuaTeX, DVI-LuaTeX), if the language package setting
is "auto" and there is no language not supported by Babel and no package
providing Babel.

This solves some Babel-related autotest cases and leads to some new failures
due to the polyglossia language nesting problem.