Nous parlons serieux.
You can say "Ils sont sérieux" in French to mean "they are serious."
Sérieux (if what you are talking about is masculine) or serieuse (if what you are talking about is feminine)
To say "let's talk" or "let's speak" in French you would say parlons.
parle à = talk to
talk = discours !!
When you talk loud is "quand tu parles fort" in French.
libre de parler
écris-moi quand tu le pourras
You can say "De quoi veux-tu parler?" in French to ask "What do you want to talk about?"
To say "I would like to talk to my friend" in French, you can say "J'aimerais parler à mon ami."
Je suis sérieux/sérieuse
Demain, il sera sérieux.