1. A language learning system, for assessing a pronunciation in a learning sentence, the language learning system comprising: a storage module, 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, each of the decision paths comprises a plurality of decision nodes and is corresponding to at least one feedback information, and the decision paths and decision nodes on the assessment decision tree represent a specific type of pronunciations having a plurality of predefined types of tones; a feature extraction module, configured for extracting at least one pronunciation feature of the pronunciation, wherein the feature extraction module is further configured for extracting at least one pronunciation feature of each of the training data, wherein the training data has at least one grade mark, a decision tree generation module, configured for generating the assessment decision tree according to the pronunciation features and the grade marks of the training data, a feedback information generation module, configured for analyzing each of the decision paths of the assessment decision tree according to the pronunciation features corresponding to the decision nodes on the decision path to identify incorrect pronunciation types represented by the decision nodes and setting the feedback information corresponding to the decision path according to the incorrect pronunciation types represented by the decision nodes; 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 to correct at least one incorrect pronunciation in the learning sentence.