This is a simple python program I wrote in my free time as an Electrical Engineering student, thus it will probably not be updated, and I doubt it will need to be updated regardless.
To simply and plainly explain this program, it will grab a copy of the English or Global localization, and then rename the copy and place it correctly to trick the game into running and not crashing like it usually does if the specific localization you are running on is not available.
This is a somewhat over-engineered solution to a simple problem, but it seemed necessary if you had a ton of maps downloaded as not many know of the fix, and even if you do, doing it manually is a chore.
USAGE:
Simply run the pre-compiled "BO3 Map Fixer.exe" file, then provide it with the workshop folder for BO3.
Example: C:\Program Files (x86)\Steam\steamapps\workshop\content\311210
Example: Y:\SteamLibrary\steamapps\workshop\content\311210
The program needs the path to workshop folder "311210", that is the folder for all of your BO3 maps, no matter which drive they are on.
Once you input the folder path, please pick whatever localization your game version is running (If you don't know, when the game crashes it typically mentions which one it is.), then just press "Fix Custom Maps".
Voila! All your maps should fixed in a few moments!
Enjoy!