What's more, sax is an en-word, so saxen is the definite singular form. There is a translation that sort-of transforms it into singular in English: "where is the pair of scissors" (where saxen stands in for the entire phrase the pair of scissors: what an economical language Swedish often is!) -- not that we'd typically say it that way in English.
I've never heard 'where are the scissors'. It's 'where is the scissors, or where's the scissors'. Scissors is like a hammer, screwdriver, etc. Can't see it as other than a single object. But I suppose there is no such thing as a scissor. Then there are pliers. Not much different in action than scissors, basically two parts with a hinge point. I've always heard 'where are the pliers'. Never 'where is the pliers'. Oh, well...
US native here. We always say "Where are the scissors", however, in Pennsylvania Dutch country, some older folks do say "Where is the scissors". We always just assumed it was there bad English showing through (as with much of their speech), but now I'm wondering if they say that in German, too.
Nice question. Pennsylvania Dutch originated from from a German dialect (Deitsch, Palatine German). "Where is the pair of scissors?" in contemporary German is Wo ist die Schere?, and in contemporary Dutch (the language of the Netherlands) Waar is de schaar?. So in Swedish, German and Dutch scissors are countable tools. I found a link about Pennsylvania Dutch: https://en.m.wikipedia.org/wiki/Pennsylvania_Dutch