English is definitely not an outlier. In french it's Rayon X

I had a moment to check and it’s actually interesting - looks like the world is split in two. Looking at a few languages closer to home (for me) we have:

X-ray-like: English, Spanish, Italian, French, Romanian, Turkish, Portuguese

Röntgen-like: German, Danish, Swedish, Danish, Norwegian, Finnish, Czech, Polish, Russian, Hungarian

There’s way more of each than I’ve listed, of course. I just picked a few languages

People in Turkey definitely don't use X-ray, they use Röntgen. Same for Azerbaijan.

X-Strahlen (X-Rays) is the name Röntgen invented for his discovery :)