Copy the language mappings we did in the central server.
Copy the language mappings we did in the central server.

See https://github.com/fle-internal/ka-lite-central/blob/master/centralserver/i18n/data/supported_languages.json

to understand the changes made here. Basically, different translation
sites have varying levels of language contribution. Some sites have a
more complete `es` translation, some sites mainly use `es-ES` instead.
This mapping is introduced in the MakeFile to make sure we use the more
complete language used by the translation site.

fle-internal/content-pack-maker