What is “C” format?
For locale English (New Zealand) the first 3 are already default and if I switch to locale English (UK) or English (Australia) the first 3 are still default.
The C locale is the standard POSIX locale, language-country agnostic and originally very limited even in the supported charset, hence usually given as C.UTF-8 instead. It’s basically the locale the C compiler has to support.
And no, there is no such C locale in LibreOffice, if LC_CTYPE=C.UTF-8 the locale used in LibreOffice still falls back to en-US.
You mean on the operating system level?
Yes, LibreOffice comes with its own set of locales, and if none matches what was set as LC_CTYPE (sic, yes, for legacy reasons) it falls back to en-US. LC_NUMERIC or LC_TIME or such are not evaluated.
By adding relevant code and locale data definitions and building from source… that’s probably not what you want, so you could file a bug as request for enhancement (RFE) and report back the resulting bug number here in the form tdf#123456.