Multilanguage ZIM are not perfectly handled

[kelson42]

For french.stacjexchange.com or esperanto.stackexchange.com, content is multilanguage: french and english.

Filename should be with mul

ZIM metadatas should have fra,eng.

Same for all other ZIM files of that kind.

[kelson42]

Same for german.stackexchange.com, italaian.stackexchange.com, japanese.stackexchange.com, korean.stackexchange.com, latin.stackexchange.com, portuguese.stackexchange.com, russian.stackexchange.com, spanish.stackexchange.cpm, ukrainian.stackexchange.com