Replace « mul » ZIM « language » metadata in the multiple language ZiM

[kelson42]

With https://github.com/kiwix/libkiwix/issues/872#issuecomment-1441240397 we have decided to not use anymore this « special » ZIM language code. Instead we want to striclty respect the ZIM specification and put the list of languages in a comma separated string.

[rgaudin]

Although the spec doesn't mention anything, it's likely that readers will only display a couple languages at first sight with the rest in an extended view.

I would thus suggest to list the languages ordered by "importance" which we can set to number of entries for the moment.

[kelson42]

@rgaudin What would be the definition of "important"?

[rgaudin]

I wrote in my comment that we can consider nb of books for now

[kelson42]

@rgaudin sorry! LGTM, we can update the spec.

[kelson42]

@rgaudin Can we fix this quickly?

[kelson42]

Specification updated