[Lukas]
So I guess Frescobaldi's way of forcing LilyPond to display English
messages (just force LANG and LC_ALL to C) has unintended side-effects
for pathnames that somehow were partly masked before by some conversion
magic that no longer happens with Guile2-LilyPond.
Yes, that makes a lot of sense: Guile2 internally knows about encoding,
so we need to "convert" the input filename from the "locale" encoding.
For Guile 1.8, the binary data is just passed on and most things are
fine. We could of course guess that the encoding of filenames is UTF-8,
but IIRC that isn't always true on Windows so we have to expect that
the locale encoding matches what is used to encode the input filenames.
I think Frescobaldi should be able to just set LANG=en_US.UTF-8, right?
Which then assumes that filenames are in fact UTF-8, so maybe breaks
some other cases? Umm...