Article 50 (and a few of the following ones) of the Geneva convention on Prisoners of War delineate what forms of labor a country is allowed to have prisoners of war perform. Us getting Qretch to translate queekish is a violation, as even if he did agree to do it without coercion it was under false pretenses.
The Karaz Ankor and the Skaven are still at war. Even in rl, the laws of war don't require you to let enemy combatants rejoin their armies during the conflict.
If there's no international agreement on the rules of war, you can't commit a war crime by violating them, although that doesn't mean it can't be eg normal crime. The difference is a question of obligation - there exists things that are, in a vaccum, moral, but are not so when you've agreed not to. For instance, the geneva convention makes explicit how much you must pay prisoners of war, but that doesn't mean that everyone who paid less than that prior to their codification was evil. Moreover, parts of the conventions specifically have outs for when the other party doesn't follow them - the geneva conventions are not a suicide pact - so the fact that the skaven are not signatories and do not follow that is also relevant, not just the status of the Karaz Ankor.