In debian we have the following attached patch resulting from bug:
When the C.UTF-8 locale is used (i.e. LC_CTYPE=C.UTF-8 or LANG=C.UTF-8),
programs using fontconfig print the message
Fontconfig warning: ignoring C.UTF-8: not a valid language tag
on standard error serveral times. For example, Emacs prints the warning
ten times during startup.
The attached patch fixes the issue for me. It is based on the assumption
that C.UTF-8 should be treated like the other locale names built into the
C library (C and POSIX).
Created attachment 132278 [details] [review]
Created attachment 132279 [details] [review]
merged into git.