Synth GrapheneOS Keyboard depends on AOSP Keyboard for language support. I am not sure about Japanese Romaji specifically, but AOSP has stopped maintaing and dropped support for many languages. Unfortunately, the only good option is using a third-party keyboard app.

      I have the same issue here. I ended up with the Google app GBoard as it has Japanese input support, although it’s bloated and a proprietary software.

      I would love to see GOS use a customized AOSP keyboard with CJK language support.

      2 years later

      Synth There is a great FOSS keyboard called fcitx5-android which is the implementation of the open-source fcitx5 im. For JP support use the Anthy plugin.

      This keyboard also supports pinyin, jyutping, hangul and etc.