Translation:Before I leave, I say goodbye.
I think an English answer of "before i go, i say goodbye" should be accepted also
"so long" should not be marked incorrect.
Who says so long.. And doesn't add (and thanks for all the fish?)
Well, nobody knows for sure where the expression so long originated from. It is funny although that next to a Germanic loan translation like from Scandinavian så lenge/länge a distortion of Hebrew or Jiddish שָׁלוֹם was proposed.
So long came to English from Malay by manging "salām".
להתראות isn't really "goodbye", it's more like "be seeing you".
But לְהִתְרָאוֹת is the most idiomatic expression when you leave, like "goodbye" in English. It sounds like a coinage following German "Auf Wiedersehen" or French "Au revoir".