Learn Portuguese (Portugal): O nosso amigo é calmo e nunca diz nada. - Our friend is calm and never says anything.

Yes, double negatives are completely normal and required in Portuguese! Here, nunca (never) and nada (nothing) work together. They do not cancel each other out like they do in English. So, nunca diz nada literally translates as never says nothing, but means never says anything.

If this is a negative sentence, why is there no não (not) in it?

Because nunca comes before the verb (diz), it already does the job of making the sentence negative, so we do not need não. If the sentence just meant he does not say anything, we would use não before the verb and nada after it: Ele não diz nada.

Why is there an O before nosso amigo?

In European Portuguese, it is standard rule to include a definite article (o, a, os, as) before possessive words like nosso (our). So instead of just saying our friend, you literally say the our friend (O nosso amigo).

Why use é for is calm instead of está?