You need to find and use extra fonts which have those characters/glyphs.
The default font (Droid Sans) as well as the vast majority of fonts you'll find will only support Latin characters.
There are, of course, many many which do, so the solution is to gather a selection of suitable fonts that have the glyphs you need, Noto is one such example.
An important note: very few font files that actually contain multiple writing systems in them, since the file size quickly becomes ginormous (which can affect load times).
So you'll need to individually pick each language family you need, apart from Latin.
A great resource for this is Google Fonts. Practically all of the fonts there are free for commercial use. And you can mix and match fonts as you please, as we push the actual work of figuring out which font to use onto Godot.
By using the Fallback array in your Custom Font > Dynamic Font, you can specify as many different writing systems as you need. It's a little easier once you see it:
I hope that it helps!