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.