For anyone still having this problem, check this out. Sometimes, the local Steam .ncf files can become "confused" (for lack of a better way to put it) after a language switch or a patch. I recommend the following:

Go into your C:\Program Files\Steam\Steamapps\common\Call of Duty Black Ops folder and delete any file or folder, i.e. - BlackOpsMP.exe or the \main\ folder.

After that is done, verify the integrity of your local cache.

By deleting the file and THEN verifying, it will prompt steam to rebuild your local .ncf file (which contains the local storage information for the game) and will re-download the most current files, as well as the file or folder that had just been deleted.

Hope this helps