Klartext reden — to speak plain text. Klartext is also a message that is not encrypted, so that anyone can read it.
Tacheles reden — to talk tachles, a word that came into German from Yiddish. Duden defines it as saying your opinion openly, without false consideration.
Butter bei die Fische! — butter with the fish! It is a regional call to stop holding back and settle the matter now.
Ross und Reiter nennen — to name the horse and the rider. You say openly whom and what you mean.
Um den heißen Brei herumreden — to talk around the hot porridge. It is a complaint about someone who does not dare to raise the real point.
Das geht gar nicht — that does not work at all. It is the everyday verdict on something that is plainly unacceptable.