Quislet was an energy being from a dimension of such. He basically flew around in a special little spaceship all of the time, and had the power to leap out of it and inhabit inanimate matter, thus animating it as illustrated here, in what is possibly the best example of someone's power in this whole collection of images. That robot looks sassy! Quislet tended, in fact, to create pretty fun shapes while inhabiting things. The other aspect of his power (not mentioned above, for shame) was that once he had occupied some matter for long enough it disintegrated, which I always thought was a neat little caveat. I don't know that it really featured very much in the stories, though... I think that maybe he destroyed a vital part of some super-villain's evil machinery once by inhabiting it, but not much else springs to mind.
The closest I've ever come to figuring out Quislet's name is that it's derived from quisling, and means something like "little traitor" - which is kind of apt since he was a free-thinking outsider in his home dimension and stole his little ship from his people, but I don't know if I, in a similar situation, would advertise this pseudo-treachery whilst trying to join a super-heroic group.
Mr. Untranslatable was a "wacky alien" type of character, the kind that doesn't quite think like everyone else and watches/quotes a lot of television (presumably. It's never shown but it's implied). I found him a nice departure from the so-very-serious Legion that had developed by the time he was inducted, but I guess that others didn't think so. He eventually lost his spaceship and had to go back to his home dimension, and despite the fact that he was likely assimilated or killed by his people once he got there, basically no one has mentioned him since. Hell, he didn't even get a statue, even though it'd be more like a model kit. This, I think, shows some level of disdain on the part of Legion writers. Sad sad sad.
Quislet is JOHN APPROVED, while his treatment is NOT APPROVED.