Como dizer "Papo reto! Na moral - sinistro..." em inglês

Papo reto! - I am dead serious (while saying that...), before a candid conversation or meaning that you will say what you mean, no matter what.
It can also be "I am going to call a spade a spade" or 'i am serious/I am dead serious...!" (related to "real talk/serious talk" (papo sério).
Sometimes it means "no kidding", and other times just a "filler" (something with no translation, meaning you will say something afterwards).

Na moral - honestly, frankly. Etc

But in the Jota Quest song it seems it meant "no sapatinho" (na acepção de "ficar de boa", "ficar suave" (meio que "enjoy it", enjoy the song...

Sinistro - awful (no sentido de muito bom), wicked/wicked cool ! (idem, 'muito bom, muito legal, maneiro!)
To describe how great something is, as in "esse pisante (tênis) é sinistro!/"esse tênis é da hora!"

Ref. rioparapaulistas.wordpress
Carioca não diz obrigado! Diz valeu! LOL, Not always, be aware of your audience, the place you are, age (people's generation), and so on. But "valeu" is widely spoken around Rio, in informal ways.

Rio may be quite slangy sometimes, even academic people like Patricia "Patthy' Silva (a "preta de Rodinhas") in YouTube videos and podcasts may go slangy like that depending on her audience, the person that has invited her to talk. interaction at play!