I recently installed the latest LTS Ubuntu (18.04). I tried it, and installed Japanese keyboard (in addition of the French one). It worked perfectly, and I even had an AZERTY layout for my Japanese keyboard!
But I just installed Cinnamon Desktop Environnment, and now I cannot have a fully Japanese keyboard. If I use Cinnamon keyboard manager, I can use a Japanese keyboard, but not Mozc. On this keyboard I didn't find how to switch between rômaji to hiragana, hence, it's just a QWERTY keyboard. If I use iBus (as in Ubuntu DE), I can find Mozc keyboard, but iBus works alongside of Cinnamon selector, so it's crap.
So, how can I have a functional keyboard selector (that I can change with super+space shortcut), that allows me to write French in AZERTY and Japanese with hiragana (with rômaji input on QWERTY or AZERTY layout, last one would be better, but I'm used to use QWERTY for Japanese in Windows).