Hello there! I think, that it will be useful - add button called by right-click mouse "Translate". User can make translate one or more words/sentenses without translate all page.
Maybe it will be better to make little pop-up window, that say definition of translated word in OS installed language. For example - OS installed language is Spanish and user wants to translate English word "meal" to Spanish.
User highlited it and use right-click on mouse or long-tap on Android and see pop-up window, that translate word and describes it in Spanish. (like in popular translating services)