russo
товарищ по переписке português
amigo por correspondência espanhol
penpal catalão
amic per correspondència 
One of at least two people who regularly write to each other, particularly via postal mail.
1
Net.Plus: Did you ever have a pen pal when you were a kid?
2
But don't start writing that interstellar pen pal quite yet.
3
To take their minds off the struggle, they decide Zach needs a pen pal.
4
Pardesoteric has begun to feel like a digital pen pal.
5
One of the big reasons I'm writing is to get a... a pen pal.
1
She has been a devoted pen friend to Johnny L. Robinson since then.