Georg Seifert:
Glyph names are converted to be more designer friendly (e.g.: afii10017 > Acyrillic)
Is it a necessity at all to touch the glyphs filenames? There's contents.plist for mapping any desired glyphs'
names to their actual
filenames. And this is UFO's built-in solution for such issues. It gives designer or developer a freedom with naming glyphs while keeping the filenames intact. Also, it saves developers problems with not so precise recommendations or conventions for filenames, such as you have faced here.
Georg Seifert:
Than the language filter in the font view and lot of other things do not work.
I think that most reliable way for language filtering would be glyphs' unicode points, not filenames. Although this rises a problem of alternative glyphs, which don't have unicode assigned by nature, but I don't think that filenames are the best solution. Perhaps this is the place to follow OpenType's convention of alternate signature being added as .suffix to the glyph name. Designers are used to that, this could be quite intuitive and, what's important, without any affect on filenames.
Regarding rewriting XML contents, I will make few more simplified tests and I'll send you some files later today to check out. Thanks for your interest, anyway. It's a good work you're doing here.