Huh, spaces. There's way too much software, especially on Windows, that breaks when there are Cyrillic characters in a path. I'll let you guess how I found out.
The double grave I is used by some obscure orthodox religionious texts
Fortunately the future of the Windows API does look better since Microsoft has now added proper UTF-8 support since Win 10 1904. All you have to do is request it in the application manifest and the A functions will accept and return UTF-8.
Apple OSes use something they call "unichar" inside NSStrings. I'm not 100% sure what it is, but it feels like it's the same 16-bit wide character.
They really should have gone with WTF-8 [0] since the W functions generally accept WTF-16 and not just the valid UTF-16 subset.