You're never going to be able to save the unwise from themselves. But there are some things you can do.
Your browser already knows what languages you speak (because you can tell it). So:
If you have a domain that uses glyphs from a language you do not speak, it could appear differently (color, font, or accompanying icon).
If you have a domain that uses glyphs from a mixture of different languages, it could appear differently (different color again, different font again, another accompanying icon).
In neither case do you actually need to break punycode.