1. A method performed by a data processing apparatus, the method comprising: accessing a word level pronunciation lexicon and a word level training text corpus for a natural language; segmenting, using a word decomposition system, the word level training text corpus into sub-lexical units; training an n-gram language model over the sub-lexical units to produce a sub-lexical language model; constructing, using the word decomposition system, a word to sub-lexical unit mapping transducer; constructing a word level language model by: obtaining a result of composing the mapping transducer with the sub-lexical language model, and performing a projection on the result of the composition of the mapping transducer and the sub-lexical language model; constructing a speech decoding network at least by composing a context dependency model with the word level pronunciation lexicon and with the word level language model; receiving an audio stream from a user; and recognizing the audio stream, using the speech decoding network.