nodata told you. Highlight characters not expected in my locale. My locale right now is en_US.utf-8. In security-sensitive contexts like file names and domain names, it's not actually that hard to figure out which characters are a surprise for me. And if I have a file that is named entirely in non-ASCII-subset characters... which I do... then light them all up. It's OK. It won't be hard for users to figure out what's going on, even if only subconsciously.