My Oath of Vengeance paladin knows common to speak with his friends, and infernal, orcish, and undercommon to shit-talk his enemies
Unpopular opinion, undercommon and thieves cant should be the same language
Especially in settings like waterdeep where the Underdark is so intermingled with the criminal element that one of the city’s biggest crime bosses just has a well that leads to it in the middle of his public front bar.
Under common is the lingua franca of the Underdark a vast cave system. A trading pidgin of those that live there and any brave surface soul who trades there.
Theves cant is class ability a coded message telling rogues what nasty criminal things are to do. It’s there to make the roleplayers feel like a spy, con artist, or planning a heist.
The whole point of thieves cant is that it seems like ‘normal’ speech to outsiders, though. It just hides illicit meanings within, while sounding like very bland/innocent common (or whatever is used as the mask). Undercommon is immediately noticeable as foreign and pointed out.
