Andrej Karpathy's most recent presentation showed how his team trained a custom detector for stop signs with an "Except right turn" text underneath them . How are they going to scale that to a system that understands any text sign in any human language? The answer is that they're not even trying, which tells you that Tesla is not building a self-driving system.
Even so, it is quite possible to train for this in general. Some human drivers will notice the sign and will override autopilot when it attempts to stop, this triggers a training data upload to Tesla. Even if the neural net does not 'understand' the words on the sign, it will learn that a stop is not necessary when that sign is present in conjunction with a stop sign.