1. A language learning system, for assessing a pronunciation in a learning sentence, the language learning system comprising: a storage module, is configured for storing a plurality of training data and at least one assessment decision tree generated according to the training data, wherein the assessment decision tree has a plurality of decision paths, and each of the decision paths comprises a plurality of decision nodes and is corresponding to at least one feedback information; a feature extraction module, is configured for extracting at least one pronunciation feature of the pronunciation; and an assessment and diagnosis module, is configured for determining a diagnosis path corresponding to the pronunciation among the decision paths of the assessment decision tree according to the pronunciation feature of the pronunciation and outputting the feedback information corresponding to the diagnosis path.