Sure

Doesn't matter . Once you have enough keys and know the keymap you're using, the key caps themselves (should) become irrelevant, anyway.

The missing keys (mostly) affect the mapping of certain "additional" letters (from the English alphabet's standpoint) depending on the language. The BT keyboard in the IGG images has 10-10-9 (1st-2nd-3rd row) character keys, where some languages have 12-12-11 character keys. For writing fluently (say e.g. German, Finnish, Swedish, Norwegian, Danish... others, too) you'd need maybe 11-11-9 character keys, at least.

You could use some logical key combos: Ü: Alt-U, Ö: Alt-O, Ä: Alt-A... (or maybe Fn instead of Alt or both, take your pick) but what to do with Å, then? In Finnish Å isn't really used, except Finnish & Swedish share the same layout and Å is pretty relevant in Swedish. So it's not so obvious in all cases and I don't think other languages are that much easier with the given number of keys on the BT keyboard.

TL;DR: As I have no idea how some "special" characters are mapped with the IGG BT keyboard in SFOS, I was wondering if the keyboard still remains usable in other languages for mails/note taking/etc. ATM, my rough guess is "not very" (but this is not to blame chenlianghen or the Youyota team, for that matter). FWIW, in terminal/SSH/(light) coding use I'm sure the US-QWERTY keymap in question will do just fine.