Sentence:  Jubal 's conversations coming and going - Jubal 's words troubled him most ; he studied them , compared them with what he had been taught as a nestling , struggling to bridge between <WSD>languages</WSD> , the one he thought with and the one he was learning to think in . 
Meaning of the word marked with <WSD> tags:  a systematic means of communicating by the use of sounds or conventional symbols