We use the spelling datum/data. We pronounce it 'dar-ta' not 'day-taa'. I want to say we pronounce it 'day-tum' not 'dar-tum' but both sound fine to me, possibly thanks to all of the exported American English we consume or possibly just because we're weird.

I appreciate the apparent strangeness of this, but it's certainly not unique within the language.

That's English! :)

uh.. I don't think dartum is used, I've never heard it. "A piece of data" maybe.. But it's rare to have to talk of just one.