BCP 47 language tag

[ArmorDarks]

• [ ] Add information about on which specs based our locale, since it isn't ordinary Gettext format, but more modern and used in browsers BCP 47 specs.

  Despite we are using node-gettext, which suggests that we should use Gettext format for locale, node-gettext actually does not care, while for web it is more relevant to use BCP 47 format.

  See for details my post here: https://github.com/foretagsplatsen/numbro/pull/255#issuecomment-318853794

• [ ] Go through code and ensure that we handle most often occurring formats of language tags correctly.

  It should be handled correctly, since I designed at first place our utilities based on that format without actually realizing that it was BCP 47 specs, however worth checking. For instance, we might need to add helpers to get script, or language without script.

• [ ] Tell that helpers returns language in ISO 639-1 format, while region in ISO 3166

• [ ] Specify that ISO helper is actually returns not exactly ISO, but Gettext format