English Translation
We’ve been friends for a long time.
How about “Nous étions longtemps amis”?
We’ve been friends for a long time.
How about “Nous étions longtemps amis”?
That sounds to me like you are no longer friends.
We’ve been friends for a long time.
or that the friend is passed away (dead)
To me, both sentences give the feeling that the friendship is over.
I would have translated the english sentence as “Nous sommes amis depuis longtemps” or “On est amis…” depending on the context. I could also see “Nous avons été amis depuis longtemps”, adding “depuis” to reinforce that it is still ongoing, but I think this one would be used if you what to contrast the fact with something else (eg, but lately we dont see each other as ofter anymore), whereas my sentence is more of a general statement.
I am sure there is some more grammatical explanations to be given regarding tense and aspect and whatnot, which would explain the difference. But I think that, in this example, without the context, to enable us to add the correct bits to convey the exact meaning of the original sentence, some information is bound to be lost in translation