Kiara: knows three, teaches german whenever she can.
Reine: speaks three, teaches ID on stream.
Lui: Speaks 2 teaches JP to EN.
Subaru: The only JP that learned english a few months.
Mumei:
How did 4/5 of the group become so good at it?
Birds in nature have a complex communication system with grammar and everything.
Image credit:
holobirds | Jazz Jack #pixiv
https://www.pixiv.net/en/artworks/94382848 