I've tried to type in non-ASCII (Ukrainian) and preview shows it correctly. Does it guarantee it would print it properly?

Yes! You can thank my mom for that :) She wanted a shirt with some extra unicode characters yesterday so I added support for that.

	▲	joombaga a minute ago \| parent [-]
		Your site supports Hebrew, but it looks like Printful doesn't for most fonts. They all come back with placeholder characters. The letters do show up with at least some of the fonts if I design directly on Printful.