How do you say 'friends forever' in Japanese?

Answer 1

You can say 一生の友達 (isshou no tomodachi) to mean "friends forever". Its literal meaning is "lifetime friends", and this makes sense when you think about religion in Japan over time, which has had little to do with a reunion-based afterlife and more to do with Buddhist/Hindu philosophies, thus two people being "friends forever" would be seen as impossible and the term "lifetime friends" is used.

Answer 2

"Eien ni yuujin" would be a way to say it.

"Itsumademo yuujin" is another way.

You can insert 'tomodachi', 'tomo' instead of yuujin (yuujin casualy means very close friend, best firend, chum).