I’m hoping to find a sort fo canonical per-language answer whether
polyglossia is a better choice when typesetting with lualatex. This is a discussion of them in general, this is too short and obviously refers to 2012, since when a lot of development has happened.
So: assume I have the correct OpenType fonts. Lualatex is on, fontspec is prepared. I’m going to type [insert your favourite language here]? Do I prefer babel? Or polyglossia? Or something else (obviously, I choose luatexja for Japanese, but what for Simplified Chinese? For Traditional?) Possibly this could be a Community Wiki.