Fixes portuguese translation

Fixes mostly typos and inconsistencies:
- "Apagar" and "Deletar" were both used. In pt-PT, "deletar" is not a thing.

- "mensagem" (Message) was mistyped a few times

- "plaintext" was translated both as "não cifrado/a" and "texto simples".
  Decided to use the former everywhere, as the latter sounds like a literal translation.