I've been inputting Devanagari with a 30% ortho keyboard lately using QMK. ऐसे अति मुश्किल नहीं है । I have an extra layer for nicher inputs just as I do with my usual Roman text. I'm not aware of any writing systems that normally require multiple layers. The most complex I can think of is Hangul, and they all should translate pretty directly to a smaller keyboard.

Any language that has more than 30-35(?) letters needs layers.

Even french and german need layers (shift or right alt) for all the stuff like ß or adding accents like é è ê ë