> ... and will always use ASCII for most kind of source code files

Same. And I enforce it. I've got scripts and hooks that enforces source files to only ever be a subset of ASCII (not even all ASCII codes have their place in source code).

Unicode chars strings are perfectly fine in resource files. You can build perfectly i18n/l10n apps and webapps without ever using a single Unicode character in a source file. And if you really do need one, there's indeed ASCII escaping available in many languages.

Some shall complan that their name as "Author: ..." in comments cannot be written properly in ASCII. If I wanted to be facetious I'd say that soon we'll see:

    # Author: Claude Opus 27.2

and so the point shall be moot anyway.