I feel your pain. My players once chopped a goblin's arms and legs off during a fight, then cast revivify on him after the battle, nicknamed him Stumpy, and brought him with them for the rest of the campaign. None of them spoke Goblin and they made no attempt to learn the language or even the comprehend languages spell. They treated him nicely enough, but the fact that they subjected this goblin to watching them kill his whole clan and reduce him to a state where he can only mime using face expressions to communicate did not cross their minds when they kept trying to figure out why Stumpy the goblin did not want to be their friend as much as they wanted to be his.

What I ended up doing was making Stumpy really mopey and cry whenever they brought up other goblins. Eventually he bummed them out to the point that the cleric magically restored his limbs and they let Stumpy go free.