Darkspear Trolls are offered to take part in rise of Zandalari, when Vol'jin refuses to join them he hears 'Voli'jin of the Darkspear, you would turn your back on own people'?
So yeah, he turns his back on Zandalari, and darkspear as a horde go to zul'aman/zul'gurub and kill other trolls. Never seen anyone complaining or fiding it strange. Pandas killing pandas is kinda the same deal. They are same race, they may refer to each other as 'own people', but they have different alliagances and will kill each other.
As for the languange barrier - it's just matter of gameplay, really. Why can't blood elves speak to alliance? They have been part of it for ages and they can speak common better than orcish.Why can't goblins speak with alliance? They make deals with everyone, so its pretty obvious they have ways to communicate with other faction. Why can forsaken, former human, speak common? Why NPCs of all races can communicate with other faction when they give them quests? Language barrier has nothing do to with lore, its just matter of gameplay.
Also why in rated BGs or arena we can't speak with members of same faction? Killing members of same faction/race in rated bgs and arena was never confusing to anyone, why now people think they will have problems with saying which panda is horde or alliance? You won't, dont worry.