Don't always "succeed" in wxUILocale::UseDefault() and, consequently, in
wxLocale::Init(wxLANGUAGE_DEFAULT), under Unix systems, even if the
locale couldn't actually be set, as it can happen if the environment
variables contain a locale which is not supported on the current system,
e.g. a "mixed" locale such as "en_FR", or even a completely invalid
string such as "bloordyblop", which still used to succeed.
For now only fix it for reasonably modern systems with locale_t support,
it could be done even for the ancient ones without it later too if
anybody still cares about them.
Closes#23218.
2ca76449bf