According to Glyphs a lot of letters were missing in some languages (pic 1).
After adding nearly all of them – except IPA – (pic 2) the number of languages covered hasn’t increased a single bit (according to @mekkablue’s script ‘Language Report’).
How is that possible?
Follow up question: Which languages do ‘Adobe Extended’ and ‘Extended’ currently cover? Is there a conclusive list to look up somewhere?
THX!
I believe the script does not check for Cyrillic scripts, so adding Cyrillic glyphs does not change the number of supported languages according to the script.
That might be the case. Still I also added lots of latin characters as well but no change there.
The script focuses on Latin, contains a summary from multiple sources, the output explains exactly what is still missing for supporting which yet unsupported language.
Same font shows following results:
Font Explorer: 146 languages
Fontdrop.info: 132
Schriftsammlung MacOS: 172
Script ‘Language Report’: 224
https://hyperglot.rosettatype.com: 560
Again how is this possible? What are the parameters?
We had this discussion a few times here already. There is no general binding rule what constitutes a necessary minimum charset for language X. So different apps wll have different databases, unless the stole from each other.
It’s very hard to say when a font supports a given language. If you have an ä but not á, can you say that you support German? So every app has different “rules”. You can also take a look at Alphabet Type tools, there is a nice overview with auxillary characters and a language checker that will give you yet another different number 
Very nice tool indeed, thank you. I would need a definite number which I can tell our customers but I could as well turn a wheel of fortune here.
