Currently FMOD Designer doesn’t have a specific facility for localisation. You might have some success using the project property "Audio source directory" to choose different source paths and "Build directory" to choose different destination paths.

The way we do this is to have one project for sfx, and one project for each language that we localize to. The game loads the sfx project and the project for the language it’s currently using. We then reference everything by system relative paths, and it seems to work well.