"Below zero degrees?"
Translation:מתחת לאפס מעלות?
Probably in part because the value is represented by אפס and the intervals along the scale against which it is located or compared (as in talking a measurement) is represented by מעלות. Also, it's an entrenched, conventionalized pattern to ask for example how many degrees something is (on a given scale), so that might factor into using the same form or construction is used in communication specific measurements. Finally, like airlibre said, although we might say "one degree" the notions of אפס and zero are somewhat peculiar and unique (compared to other values). Many other things like mass nouns and mass noun construals can have interest exhibit interesting patterns as well.
This is really crazy, what you write seems to be true and I just can't find out why. Zero is of Arabic origin but it was originally an adjective. My only guess is that this is because it is allegedly an even number. Maybe this use found its way to most languages due to a strong lobby of mathematicians, who wanted this to be reflected.
I like how you think, but no: it has nothing to do with zero being even. The reason is that plural doesn't mean many, but not singular (or not any of the numbers the language already marks, like dual, trial or paucal). Most times, plural matches perfectly with the meaning of many, but it also marked fractions before the existence of the idea of zero.