The Polish version of 'don't look a gift horse in the mouth' is 'don't check the teeth of a horse you received as a gift' ("darowanemu koniowi w zęby się nie zagląda") (source).
Similarly, devil's advocate seems to be the same thing in Polish.
And, as far as I know, these idioms don't appear in the English translations. Unless someone read the books in Polish and recognizes these expressions, I suppose they are indeed variations to give an "otherworldly feel" to the TV series.
Moreover, I can imagine that 'playing the devil's advocate' could evoke unwanted associations, and potentially throw off viewers.
I'd like to add that I don't think one would tell the age of a cock by its beak. It might have been implemented as an additional layer of humour, since the one talking about it is Yarpen Zigrin, a dwarf, too short (at first glance) to be able to look a gift horse in the mouth.
Similarly, as 'devils' in the world of The Witcher are common entities, synonymous with sylvans (like Torque in Episode 2), 'playing priest's ear' makes a lot more sense in-universe.