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