I’m creating a typeface with the full latin Western European glyph set — but what languages does that actually cover?
There is no easy answer to that question. The glyph set should enable you to write in pretty much all living codified languages typically associated with Western Europe, including languages like Occitan. Feel free to dig around in past discussions or make your own research:
http://www.eki.ee/letter/
http://www.omniglot.com/language/names.htm
https://www.underware.nl/latin_plus/
https://elen.ngo