La nuit tous les chats sont gris

Swedish ? I mörkret är alla katter grå
Dutch ? In de nacht zijn alle katten grauw

Thanks for the explanation everybody, I've never heard this expression before. / Grazie a tutti per la spiegazione, non ho mai ascoltato questa espressione prima.

greg: io ci sono volentieri ;) Sono contenta di vedere i soliti noti :)

In German "Nachts sind alle Katzen grau" is far more common.

It is night where I live right now, so I guess all the cats are gray.

I've seen a couple of translations of this in Japanese. One was kind of gruesome.

Korean:

(latinized for convenience)
Bame, mōdun kōyanginun hōisækie bōinda.

Slovenian:
Ponoči so vse mačke črne.

A speech from a communist apparatchik in Georgia (long time ago):

"It is very hard to catch a black cat in a completely dark room.
Specially if the cat is not there.
But we shall catch it!"