> Not sure it's related, but there seems to be a bug:

It's not a bug, it's a feature :-) If you specify a non-existing locale, the 
GNU libc fails back to ascii.

$ locale -a
C
français
french
fr_FR
fr_FR@euro
fr_FR.iso88591
fr_FR.iso885915@euro
fr_FR.utf8

$ LC_CTYPE=fr_FR.iso88591 ./python -c "import locale; 
print(locale.nl_langinfo(locale.CODESET))"
ISO-8859-1

$ LC_CTYPE=xxx ./python -c "import locale; 
print(locale.nl_langinfo(locale.CODESET))"
ANSI_X3.4-1968