Use ISO 639-1 language codes instead of ISO 3166-1 Alpha-2 country codes

[veqtrus]

Current: https://en.wikipedia.org/wiki/ISO_3166-1#Officially_assigned_code_elements
Proposed: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes

The localisation README (and wiki) require using ISO 3166-1 Alpha-2 codes. In practice this is already not followed by en (English), cs (Czech), ja (Japanese), as well as variants of Portuguese and Chinese.

To be accurate we could use one of the https://en.wikipedia.org/wiki/Language_code options. The IETF tag is probably the best since it's already used by Facebook/ locales-config.json.

To do this we will need to rename some files and update locales-config.json.

[veqtrus]

Implemented in #3150