First thing that comes to mind (as a GUI developer that is currently translating a product) is that the text renderer can't handle accents above and below Latin characters. (e.g. Let's buy crème fraîche in Curaçao) I'd also be surprised if it can handle right-to-left strings as well.
Flutter uses Skia for text rendering and layout; the same rendering engine used by Chrome, Android, and others. I'm beyond confident it can handle all of those situations.
Flutter demos have a localization dropdown for selecting different locales, of which the gallery demo at least supports many, well beyond FIGS: https://gallery.flutter.dev/#/
