Unfortunately, no. In short:
OSM Planet → split into tiles → translit → Mkgmap → Garmin tile
I’ve investigated further and I’m pretty confident that the translit application is to blame here, it’s supposed to (among other things as explained earlier) add a ‘name:zh_py’ tag (Chinese Pinyin) to Chinese roads/nodes but apparently does this also for Japanese roads/nodes. This is clearly a bug.