feel i think it may be the plutonium as u said, japanese characters are too difficult to write in the game, as same as these files, changing it in a mod like plutonium probaly would get some errors, it doesnt happen with bo2 because its a different way to install, u install bo2 without needing steam to do that, maybe the problem is happening because mw3 had to be install from steam, and some archives can conflict with each other, so keep it in mind and try to talk with some plutonium dev and he should be answering ur questions
(sorry for the long text)