Hey Cyril_LeGwak, welcome to the forums!
I just wrote you a PN, sent you a alpha version of the Modlauncher I wrote. It is able to switch the game language, I think the issue in your case is that the Game tries to start the mod in a Language that is NOT English, and thus searches for sound files that dont exist.
Please be aware that my application is in early Alpha phase and many functions are not finished yet and very VIP.
Once you selected a Mod to start with it should remember your preference and you than can use it to launch TPC in the right Language.
However, now that you switch Language from (I guess) French to English and (I suppose) your System has French sound files installed that would enable the Mod sound, but disable all Vanilla Sounds.
There is a workaround for that, I would need to rename all english Sound files from _eng to _fr.
I am going to implement that feature into my program in the future if needed.
Further, you could choose to keep the French Vanilla sounds instead of the Mod ones, but then you would need to go into ./Forces of Corruption/Mods/Staragte/Data/Text and to manually rename MasterTextFile_ENGLISH.dat to MasterTextFile_FRENCH.dat or to the Language your Game has insalled. But that would only remove all MISSING text from the game but the mod sounds would still be missing.
Which OS do you use by the way?
Anyways, please tell me if it helped.
Also thank you for entering our forums, any feedback or bugreports regarding the Mod is highly welcome.