Marius,
This mail just reminded me that one of my own projects didn't handle a messed locale setup gracefully in the past causing multiple users (that apparently had a messed setup) to report issues, e.g. https://aur.archlinux.org/packages/ syncthingtray#comment-816671 and https://github.com/Martchus/syncthingtray/ issues/109. And when I searched for the error to investigate the issue I found many results. If it helps preventing applications from crashing completely with errors like "locale::facet::_S_create_c_locale name not valid" it is likely worth including.
fwiw, for people writing code: a friend suggested something like this: first try user's configured locale (environment variables). if that fails, try the "C" locale. if that also fails, print out a warning, and continue to run: ---- /* modified from https://www.cl.cam.ac.uk/~mgk25/unicode.html */ if (!setlocale(LC_CTYPE, "") && !setlocale(LC_CTYPE, "C")) { fprintf(stderr, "Warning: Can't set the specified locale! " "Check LANG, LC_CTYPE, LC_ALL.\n"); /* but, keep going */ } ---- (though, of course, "continue to run" may not work in your case, i.e., Boost or other frameworks, etc.) cheers, Greg