1) to speak according to one's lights
2) not to mince matters

not to mince words

to be outspoken

plain talking

"wie ihm der Schnabel gewachsen ist" = speaking in his mother tongue, NICHT "er sucht nicht erst nach richtigen Worten".

to call a spade a spade

to be outspoken is to be "vorlaut"

Er redet, wie ihm der Schnabel gewachsen ist:

he talks (unashamedly) the way he learned it
he doesn't mince matters
he calls a spade a spade

(these are examples from Langenscheidts Handwörterbuch)

He doesn't beat about the bush.

Didn't the German expression also imply that one should should use plain language and avoid the stilted Hochdeutsch? Isn't this in a famous Goethe quote?