1. A non-transitory storage medium storing instructions readable and executable by an electronic data processing device to perform a method including the operations of: optimizing a penalized loss argmin W ƒ(W)+λΩ(W) using a corpus of language samples respective to a set of parameters W={w v } vεV of a language model, where V is a vocabulary, ƒ(W) is a function measuring predictive accuracy of the language model respective to the corpus of language samples, Ω(W) is a penalty comprising a tree-structured norm, and λ is a regularization parameter; and outputting a trained language model with optimized values for the parameters W generated by the optimizing.