"Op een weegschaal op de maan weeg je minder."
Translation:On a scale on the moon you weigh less.
Me neither. I suppose it is technically wrong to say "A scales", but I think of it as shorthand for a set of scales, or a pair of scales. Modern weighing machines aren't literally sets or pairs, anyway (like the Libra sign of the zodiac, or those held by Justice). Yet to say "a scale" (singular) just sounds wrong. If it were possible to weigh oneself on the moon, I'm sure one wouldn't use classical scales anyway.