treequell 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.
boarim 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.
stupidcreature 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.
kelp I'm trying out fcitx5i-android. I was about to write for help cos I installed it but couldn't use Japanese. Just so others don't fall into the same trap, once installed, I had to go into the app and enable the Japanese keyboard before I could actually type. Obvious once you know.
boarim kelp How to do that? I can’t see the option in the app. On GitHub, it says that Japanese IME is provided by the Anthy-plugin but I don’t see it in the app (I downloaded from the Play Store).
stupidcreature boarim You need to download and install the Anthy apk from the github repository. Here is a handy link for the current version at the time of writing this: https://github.com/fcitx5-android/fcitx5-android/releases/download/0.1.1/org.fcitx.fcitx5.android.plugin.anthy-0.1.1-0-g3f41b65d-arm64-v8a-release.apk