Now it is accepted:
"You suggested “keep calm” as a translation for “Bądź spokojny.” We now accept this translation. :)
Thanks for the contribution, please keep it up!" (c)
Although it's not really exactly the same; "Keep calm" implies that the addressee is already calm, and is being invited to remain so.
OTOH, "Be calm" (which, actually, would be considered perhaps a little non-standard), and the somewhat better (and already accepted) "Calm down" both imply that the addressee is definitely not calm, and is being invited to become so....
Roughly the same number of people that responded positively to "DON'T PANIC!" (despite all the Large, Friendly Letters)...
Doesn't seem very new ;) Anyway, it has a different meaning, closer to 'cool', 'okay''. But not 'calm'.
Maybe it's a regional difference, but for me that works only with repetition(ie. „Spoko, spoko [cośtam]”).
A noun or an adjective with noun would change to instrumental, but an adjective alone stays in nominative.
bądź spokojny - Czy mogę pisać ’be quiet’ zamiast ’be calm’ czy nie? Can I write ’be quiet’ instead of ’be calm’ or not? Dzięki.