If you have the error:
ERROR: sound bank failed to load mpl_patch.all
Please do the folllowing in order to fix it:
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Go to Steam library and on Call of Duty Black Ops II - Multiplayer select properties then go to local files and click on "VERIFY INTEGRITY OF THE GAME CACHE"
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Wait for Steam to download the missing files
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Reapply the file Localization_mp from the language patch and it will fix it