Hi Ceex!
Absolutely, the internal engine will support Japanese in the future. However, you can already use Japanese in GameTranslate if you use the DeepL engine.
Register a free account at https://www.deepl.com/en/pro-api, get your API key, go to GameTranslate and insert it into the "DeepL API Key" slot after selecting DeepL Free as your translation engine.
Let me know if you get it working! :)