Only I'd like the translation to be "Who will save the saviours themselves?" / "Who will save the saviour himself?" / "Who saves the saviour?" (you get the idea).
"Saviour" in this context is not a religious reference. It refers to a character who rescued his friend from an assault (if you read my previous question, Character Y). You could also go with "rescuer", or "comforter" - "Who will rescue the rescuer/s?" or "Who will comfort the comforter/s?".
So far: looked through Latin words and phrases at Wikipedia. Also tried an online translator, and got the complete rubbish quisnam mos servo saviour sui?. I thought that maybe it didn't recognise the British spelling "saviour", but when I tried the American spelling, I simply got the same thing with "saviour" replaced by "savior". I've also poked round some Latin dictionaries, but a) I've forgotten everything I ever knew about declensions, b) there seem to be a lot of different Latin verbs that can be translated as "to save" in English, and the online dictionaries aren't precise enough to give context.
Will be very grateful for help!
Edit: Answered here. Thanks!