Publish a list of used ISO639-3 code in `Language` metadata

[benoit74]

To allow readers to check they can interpret any value of Language metadata, the CMS should publish a list of all ISO639-3 codes used in any ZIM it manages.

This issue comes from openzim/overview#51 (comment) discussion

[aviralgarg05]

i will be solving this issue

[benoit74]

This issue is not ready to be worked, it is far from being clear how we wanna solve this ATM.

Note also that no CMS issue is ready to be worked on by volunteers ATM in fact, I'm doing a heavy upgrade of the codebase. Parallel work with multiple developers is not possible, too many moving parts.

I noted your interest in this issue and will come back to you once things have settled. Do not hesitate to have a look at other repos, especially issues tagged "good first issue".