Changing language interface doesn't work

I have changed the language settings from italian to english, but every time i click a blue word or a sentence, it gives me the italian translation and not the english one.
If i change that translation form italian to english, the next one i click is still in italian.

Sounds like you actually want to change the DICTIONARY language, not the INTERFACE.
You can change your dictionary language to English under the settings button (gear icon) on the home page, Reader Settings tab.
Hope that helps!