"Klaar voor de start? Af!"
Translation:Ready, set, go!
To what extent is it wrong to say: Ready for the start! Go! Or do we have to adhere to this idiomatic translation
"Ready for the start" is a gramatically correct phrase (not a compete sentence), but it would never be used in this situation. (Starting a race). You could say, "Ready... Set...Go!" (Informal). You could also say, "On your marks... Get set... Go!" (Acceptable for formal or informal situations.)