You can get additional fonts by installing them to your operating system, so macOS, Linux, Windows or Haiku, with Android this is currently impossible, additional fonts installed to Android are not accessible right now.
For my understanding, you can use each and every symbol your fonts offer, but Krita is not a word processor but a painting program, so, although Krita’s text tool is currently rewritten and modernized, it can be that you will miss some functions a word processor will have.
But you can prewrite your texts in a word processor and import them into Krita, so far I can’t remember that Krita suppressed any special character I ever used.
One thing you have to obey, is, that the current versions of Krita, from 5.2.0 to the recent 5.2.11 can not handle all fonts. You can work around this by using older versions or one of the recent Krita 5.3.x Nightly versions, which then will offer all the bits from the rewritten text tool that are already implemented, so you already can profit from them.
If you are interested in the features of the new text tool, you will find the topic where this is discussed here: Text Tool Thread
Michelist