Tú - Informal (used with friends or people much younger than you) Usted - Formal (used with strangers or people older than you- generally more respectful and polite when you don't know the person well) The conjugations for the two are different, too. Tú=eres Usted=es So the first sentence you wrote should be eres instead of es.

That is how you say Español in Portuguese.

The easiest answer is that it's used the same way in English if you break it down. nadie con quien poder hablar = "No one (with whom) to be able to speak"