Are there families of formal languages known to be truly PAC learnable?
There is a recent result on the polynomial pac-learnability of semilinear sets at LICS 2010: Parikh Images of Regular Languages: Complexity and Applications. I guess this is not what you are looking for.

You should also have a look to the paper of Clark and Thollard: PAC-learnability of Probabilistic Deterministic Finite State Automata