There was Japanese translation in the old launcher, but new launcher doesn't have an option for Japanese translation.
MCL-11125
Japanese translation missing, not showing in new launcher
Resolved
Invalid
0
The Japanese translation is not completely finished yet. If you want to help out, you can help with the translation on the Minecraft Launcher Crowdin project. Japanese is currently 99% translated and 51% approved, so it'll be surely available soon in the launcher as well!