I was asked about a Hebrew variant to "when pigs fly" recently, and I thought: what a great idea for an article! I could gather various language versions of this phrase!
Of course,
Wikipedia beat me to it. Damn it.
But their foreign list is still fairly sparse. For example, Amharic has የህልም እንጀራ, yähləm ənjära, "injera in a dream". Japanese seems void of native idioms for this but DID import from Chinese 百年河清, hyakunen kasei, which is literally "waiting a hundred years for the Yellow River to clear". And Turkish has çıkmaz ayın son çarşambası, "The last Wednesday of never-month".
Sadly Hebrew seems to have just lifted the English phrase and uses כשחזירים עפים, kshexazirim afim, "when pigs fly." That being said, certain languages have a bleedover and their phrases can be also used to doubt what's being said (like a "yeah right" or a "right, and I have a bridge to sell you" kind of response), as seen in the Spanish cuando las vacas vuelen, "when cows fly." And then this is where Hebrew suddenly has all sorts of fun phrases to call someone on their BS -- לא דובים ולא יער, lo dubim velo ya'ar, "no bears and no forest"; לא היה ולא נברה, lo haya velo nivrah, "has never been nor ever made"; באוויר גמל פורח, baavir gamal poreach, "air is flowing in a camel", and my favourite, לו היו לסבתא שלי גלגלים, אז היא הייתה אוטובס, lo hayu l'savtah sheli galgalim, az hi haytah otobus, "if my grandmother had wheels she'd be a bus."
I'd love to hear other variants on "when hell freezes over," "when pigs fly," etc., in any other language -- even other English variants, or how those phrases are otherwise used.