Directory names are based on https://tools.ietf.org/html/bcp47
This also allows languages like: Welsh/Kymrisch or Frisian
locale examples: en, en-US, nl, nl-NL
2*3ALPHA ; shortest ISO 639 code
["-" extlang] ; sometimes followed by extended language subtags
https://github.com/the-djmaze/snappymail/issues/435
https://tools.ietf.org/html/rfc1766
https://tools.ietf.org/html/rfc3066
https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
https://en.wikipedia.org/wiki/IETF_language_tag
http://www.w3.org/WAI/ER/IG/ert/iso639.htm
http://www.loc.gov/standards/iso639-2/
https://www.iana.org/assignments/language-subtag-registry
relativetimeformat.js data is based on Unicode CLDR
