I just have a custom locale for any UNIX-like systems I use. American English, UTF-8, 24-hour time, RFC-3339 dates, SI units, '.' decimal point, '_' thousands separator. I call it en_ENG, for the fictional land of Engineeristan.
That doesn't have `_` as the thousands separator. It uses ISO 8601 for date/times instead of RFC 3339 (so "2022-03-14T15:06:03 EDT" instead of "2022-03-14 15:06:03-04:00"), etc.
I just happen to want slightly different choices than it allows.
An explanation of all the valid fields can be found at [1]. Copy an existing locale, modify it to suit your needs. Don't forget to escape all characters as Unicode if you want it to support unicode. Compile & install as normal with localedef, add to `/var/lib/locales/supported.d`, run locale-gen, set the various `LC_` variables you want to use in `~/.profile` or `/etc/envionment` or whatever.
