Support an encourage region subtags for en locales

[WULCAN]

If one of this project's eventual goals is broader adoption beyond Zetkin I think it's worth broadening the regex here [...]

Zetkin happens to be a relatively simple case where two letter language codes are getting the job done for now. Most projects I've worked on have fallen into one of these categories:

1. Locale codes such as en-GB, en-US due to global scale requiring attention to be paid to dialects and other regional language differences.
2. Locale codes as above but due to tools/people imposing that structure upon projects early in order to future proof them against global scale.
3. A combination of locale codes and two letter language codes because of an early assumption that language codes would be enough, and backwards compatibility issues preventing a full migration to locale codes once the assumption was eventually proved wrong.

Personally I think it's only a matter of time before even Zetkin joins that third category. Probably happens once there's a good few big US- and UK- based orgs onboarded and then a feature gets added where the only vocab that works for it falls within the "not easily mutually intelligible" subset of en-GB/en-US vocabulary differences.

Originally posted by @henrycatalinismith in #106 (comment)