A while ago I made the launcher not honor modified files because otherwise it because an issue when we do updates.
However, to overcome this issue with users, I have the game look for specific custom files.
The post about this can be found here.
The specific file you are trying to mod is called texturejapanese.xvm, and like I wrote in the linked post, make a copy of the file and name it texture_custom.xvm, if this file is found on the data folder, it will be loaded instead of the original file.