Just let everyone be able to speak Common / Orcish already…
Barring cross faction communication is pointless anyway…
We already have Demon Hunters who can speak to each other in Demonic and Void Elves and Blood Elves can communicate in Thalassian…
In addition to that, there should be plenty of Blood Elves who speak Common anyway as Quel’Thalas was in the Alliance before…
And every Forsaken should be able to speak Common for obvious reasons.