1. A multi-lingual text-to-speech system, comprising: an acoustic-prosodic model selection module, for an inputted text to be synthesized and containing a second-language (L2) portion, and an L2 phonetic unit transcription corresponding to the L2 portion of the inputted text, sequentially finds a second acoustic-prosodic model corresponding to each phonetic unit of the L2 phonetic unit transcription in an L2 acoustic-prosodic model set, searches an L2-to-L1 phonetic unit transformation table, L1 being a first language, and uses at least a controllable accent weighting parameter to determine a transformation combination to select a corresponding L1 phonetic unit transcription and sequentially find a first acoustic-prosodic model corresponding to each phonetic unit of said L1 phonetic unit transcription in an L1 acoustic-prosodic model set; an acoustic-prosodic model mergence module that merges said first and said second acoustic-prosodic models into a merged acoustic-prosodic model according to said at least a controllable accent weighting parameter, sequentially processes all the transformations in said transformation combination, then sequentially arranges each merged acoustic-prosodic model to generate a merged acoustic-prosodic model sequence; and a speech synthesizer, wherein said merged acoustic-prosodic model sequence is applied to said speech synthesizer to synthesize said inputted text into an L2 speech with an L1 accent based at least partly on the transformation combination determined by the controllable accent weighting parameter.