I can read Japanese at a borderline N3 level. Meaning, my biggest issue is Kanji.
So I used text hooking and the Rikai-kun add-on to help me identify unfamiliar words and Kanji. Then I simply used my understand of Japanese grammar structure. Some of the sentences used jargon that I wasn’t familiar with, but I got the gist of the meaning. I seriously need more practice though.
As for the canonicity, I can think of a few possibilities, but they’re a bit of a leap.
One is that Kagari was speaking her pluralistic language that only Kotarou (Pochi) could understand, but it was voiced in Japanese for the viewers.
Another possibility is that she did know a bit of Japanese (learned during Moon route?) but didn’t become fluent until she spent some time on Earth with the others.
Wish I could pick up Japanese that fast.