Non-English characters are quite useful in comments where you're explaining Unicode processing stuff, and in regexes working with the characters, and when you're using maths notation (proper symbols in comments, Greek letters for variables, etc.), and when you're drawing boxes in a terminal. I'm sure there are many more too.
I omitted this to keep it simple (this is why I wrote non-English rather than non-ASCII, I actually am a proponent of active usage of proper Unicode symbols like ⇒, ≠, etc, and also TUIs) but yes, I would prefer a rather extended English char-set including Greek letters, mathematical symbols, pseudographics etc. These can be useful and are not much trickier than English letters. But I would certainly like to see at least a warning (I would even prefer an Error actually) if my code file includes anything related to RTL, complex character composition or non-Latin letters other than Greek.
