Translation of "Serene" in Spanish

His serene self-assurance

de la confianza en sí mismo.

And it makes you sound very serene, like you've had lots of Valium,

que hace tu voz parecer muy calma, como si hubieras tomado mucho Valium

Gone to speak with his most serene highness, the Prince Hans Adam II.

ido a hablar con su serenísima alteza, el Príncipe Hans Adam II.