Boost not caching non-latin urls
I suspect your host is not using UTF-8 for their filesystem (rather rare these days, for example, in the early 2000, I've seen hosts using ISO-8857-7 or CP-1253 encodings for their files, because that's what Microsoft Windows did). Alternatively, Plesk or FileZilla is not using UTF-8, but that would only be a problem displaying the file names and Boost should work.

If it is because of the filesystem of your host, you could try to forward-port this old patch from 6.x-1.x which would convert with iconv the filename before saving it to disk:
https://www.drupal.org/node/797136

To detect the encoding used on the filesystem, type the command "locale". It should display:

LANG=en_US.utf8


If it display anything else than utf8, such as iso8859-xx or cp-xxxx, that would probably be the source of the problem (and will require patching Boost).

Boost does store on disk the filename unescaped, e.g. it stores it in unicode, not by escaping the characters (ex: %CE%BC%CF%80%CE%BF%CF%85).

Here is a test using Greek, Cyrillic and Latin accents:
https://www.bidon.ca/fr/μπου-χου-добър-ден-éàĉç