I kinda doubt it's something widespread.
But do doubt away. :)
Please see the official guidelines: https://www.sst.dk/vidensbase/graviditet-og-smaaboern/barnet...
Why not just say "europe" directly then? Everybody knows about that, and you don't seem concerned with correctness anyway.
My children slept outside while it was below zero C. The only time they were not sleeping outside was while they were ill.. having a cold or something.
Edit: the limit is -10 C according to https://www.sst.dk/vidensbase/graviditet-og-smaaboern/barnet...
This is well known around here: