Japanese AAC user tip:
This came from a close friend of mine who communicates natively Japanese, she uses an AAC device to do so (The device she uses has 3000 picture slot thingies o this tip only works if your device has at least 48 slots)
She said that due to the lack of non-English devices/apps made for the nonverbal, she ended up having to hand type the romanized version of the Hirigana and Katakana (Like type how they would be pronounced in English, an example like "Wa" for the japanese symbol of "わ") and then she would click each box to make a sentence. Yes, this method is slow and it is time-consuming but it is decent at non-English language communication.
This was the Hirigana picture she used to help her type in the romanized versions, I don't know what she used for the Katakana (When I ask her or if she tells me, I will add it to this post)
I hope this tip helps out any Japanese AAC users.