1. A method of building a language model, comprising: at a device having one or more processors and memory: acquiring data samples for building the language model; performing categorized sentence mining in the acquired data samples to obtain mining results comprising a respective set of sentences obtained through the categorized sentence mining for each of a plurality of categories; obtaining categorized training samples based on the mining results; building a text classifier based on the categorized training samples; classifying the data samples using the text classifier to obtain a respective class vocabulary and a respective training corpus for each of a plurality of categories; mining the respective training corpus for each category according to the respective class vocabulary for the category to obtain a respective set of high-frequency language templates; performing training on the respective set of high-frequency language templates for each category to obtain a respective template-based language model for the category; performing training on the respective training corpus for each category to obtain a respective class-based language model for the category; and performing training on the respective class vocabulary for each category to obtain a respective lexicon-based language model.