[elpraga]

Another hint (that works even better then built-in solution because allows to add multiple sources from the internet) is to install goldendict.

In my opinion it is a perfect solution to someone reading books in a different language.

In my case, I combine es-es dictd dictionary, es-cz online dictionary, wictionary and wikipedia. All results show up in a pop-up windows that disappears when I move the mouse from it. Very convenient, indeed.

Hope it helps someone!