Idioms in our own language are silly enough. But here’s how the French would say, “When pigs fly!”
“Quand les poules auront des dents!” Or literally, “When hens have teeth!”
The German version is identical to ours, “Wenn Schweine fliegen können!” But whatever language you use, it refers to an event that you think will never happen. However in art, anything IS possible and this saying has inspired some whimsical art including this flying pig at the Cincinnati Airport.