I18N: Support more valid locales: language-script and language-script-region

[w3cbot]

This is a tracker issue. Only discuss things here if they are i18n group internal meta-discussions about the issue. Contribute to the actual discussion at the following link:

§ https://github.com/w3c/webextensions/issues/724

Instructions (delete this line and all following):

• check for the following labels, then remove the PENDING label, then delete these instructions

• Check that TRACKER & S:... labels are there

• Add t:... label(s) to indicate the topic (same as ids in specdev)

• If urgent add:

• add ADVICE-REQUESTED label if the WG-issue is specifically asking for i18n to advise/comment

• add NEEDS-ATTENTION label if this is an important issue

• if there's an i18n-*lreq label in the WG repo check for:

  • ...LREQ label(s) there should be one or more
  • SPEC-TYPE-ISSUE label (dark green background) should be there
  • TYPE-INFO-REQUEST label (red background) if this is a WG-issue that constitutes a request for script/language expert advice
  • i:... label(s) to indicate the Language Enablement Index category