L10n: falls back to English when the auto-detected locale is unsupported

[wehrwolfmann]

Summary

--key-language with an empty value, and "language": null in the config, mean "use my
system locale". If that locale has no translation, fastfetch printed nothing and exited 477
instead. So one config file works on my machine and aborts on a Dutch or Ukrainian one, and
in any LC_ALL=POSIX script or CI job.

$ LC_ALL=uk_UA.UTF-8 fastfetch --key-language ""     # before
Error: Language 'uk_ua' is not supported
$ echo $?
221
$ LC_ALL=uk_UA.UTF-8 fastfetch --key-language ""     # after
OS: CachyOS x86_64
...

Related issue

Follow-up to the second point in #2503 (comment) — you fixed pt there, this is the fallback half.

Changes

• An auto-detected locale that reaches error: now returns English instead of exit(477).
• An explicit --key-language value still errors, so typos and bare zh are still reported.
• Help remark mentions the fallback.

Testing

CachyOS, x86_64, glibc 2.42, gcc 15. Differential over all 533 locale names glibc ships
(/usr/share/i18n/locales + SUPPORTED), --key-language "", before vs after:

	count
identical	179
aborted before, works now	354
regressions	0

Spot checks unchanged: ru_RU.UTF-8 → Операционная система, pt_BR → Sistema operacional,
zh_CN/zh_TW → Chinese, C → English. Explicit --key-language zh / nl_NL / xx still
exit 221. "language": null under uk_UA.UTF-8 now prints instead of aborting.

Screenshots

No visual changes for supported locales.

Checklist

• I have tested my changes locally.