My new player in my game is a rogue and is looking for languages for his character. He was asking about a thieves tongue for the local guild. I'd didn't see any references in the books about secret languages except the Druids.
Has this ever been addressed? Is there a thieves tongue/cant per the rule books or maybe just 'house rules'?
There is no special language for thieves. Starting languages available are determined by race. He would need to invest a point into linguistic to learn such a language anyways.
For what it's worth, he could simply choose an uncommon language that most others wouldn't know, like Undercommon, to communicate more discretely.