I check one by one every DLC to see if it has files for each language

Great, thank you.

Languages are English, Japanese, French, german, Spanish, Italian

Did you check in-game languages for each disc?
Add-on packs don't just automatically get the language for the games they were bundled with.

I check one by one every DLC to see if it has files for each language. Borderlands games have one folder for each language, so, for exemple, the Tiny Tina DLC has an Audio folder, in which there is 6 sub folders : English, French (France), Italian, Spanish (Spain), German, Japanese, as well as a Localization folder, with the same 6 sub folders : English, French (France), Italian, Spanish (Spain), German, Japanese

It's a bit long but it is better to check thoroughly

Languages are English, Japanese, French, german, Spanish, Italian