The variations of the idiom's translations are definitely interesting. If you're curious to read more about the language variations, Le Figaro has a nice article about the idiom's equivalent in other languages (though they use the variant « quand les poules auront les dents »).
From Le Figaro [1]: « Précisons enfin que notre expression est un idiotisme et qu'elle ne possède pas, par conséquent, de stricts équivalents à l'étranger. Ainsi nos voisins anglais ont-ils l'habitude de dire «quand les cochons voleront», les Espagnols, de lancer «quand les grenouilles auront des poils» et les Allemands, de flanquer un «quand les poissons apprendront à voler». »
English translation: "Finally, let us specify that our expression is an idiom and that it does not, therefore, have strict equivalents abroad. Thus our English neighbors are used to saying "when the pigs fly", the Spaniards say "when the frogs have hair" and the Germans say "when the fish learn to fly."
Thanks; I didn't know "idiom" is "idiotisme". Nice.
The examples from Figaro all seem to have their verb as a subjunctive in French, so "When pigs would fly" - we usually say "If pigs had wings", or "Yeah, and pigs might fly". Your translations don't carry the doubt that words like "might" and "if" impart. There are hardly any/no verbs in English that still have a subjunctive distinct from the indicative, a fact that I regret.
There are some places in England (north east) where they pronounce the subjunctive of "to be", even though it isn't written distinctly; they say "If I wear a rich man". I think it's from the German, as in "Wenn ich wäre".
From Le Figaro [1]: « Précisons enfin que notre expression est un idiotisme et qu'elle ne possède pas, par conséquent, de stricts équivalents à l'étranger. Ainsi nos voisins anglais ont-ils l'habitude de dire «quand les cochons voleront», les Espagnols, de lancer «quand les grenouilles auront des poils» et les Allemands, de flanquer un «quand les poissons apprendront à voler». »
English translation: "Finally, let us specify that our expression is an idiom and that it does not, therefore, have strict equivalents abroad. Thus our English neighbors are used to saying "when the pigs fly", the Spaniards say "when the frogs have hair" and the Germans say "when the fish learn to fly."
[1] https://www.lefigaro.fr/langue-francaise/expressions-francai...