So now I finally had time to test and debug multilanguage support, I should have it working now, and have tested successfully on a combination of Japanese and English. It turns out, Civ 5 only ever scans one language file in the folder, and prioritizes the file with Language_en_US if possible. Thus, instead of having the language files separate, I had to put them all in the same file

. It's kind of stupid and might cause loading issues, but it's what Civ 5 wants...
(The previous version actually accidentally broke non-English support, so if you're having problems with that, the new version should fix it)
I updated the modpack in the OP. The steam workshop version is out of date until I can get back to my usual computer which has the modbuddy project that will let me upload

.
Anyway, to make a multiple language modpack:
1) Ensure that all your mods support all the languages, at least partially. Otherwise, you will have issues.
2) Go to steam->library->right click Civ 5->properties->language, and change your default language to one of the other languages you want to support (but NOT ENGLISH. NEVER ENGLISH.) Wait for steam to update Civ 5, then pick another language and repeat. Unfortunately, you'll need about 1 GB for each language.
[English is always included by default, but if you pick it as your language on steam, it will erase all other languages]
3) Run the new MPMPM as normal, and it should work.
If it turns out the steam procedure has issues, it can also be done by splicing together different language versions of Override/CIV5Units_Mongol.xml manually...