GetSrcLang, GetDstLang, SetSrcLang, SetDstLang, SwapLang are reusable
functions from translator to translator. So make Language struct.
GetEngineName is also a reusable function. So make EngineName struct.
Finally, let Translator to inherit those functions in these three
(Language, TTSLock, EngineName) structs. No need to write the duplicated
functions for each Translator.
