Mustangs had to be brought over for a long time and they were very unusual in Europe. Now that Ford builds a model for Europe, I see them a ton here in Hungary. It's easy to spot them because they have the license plate section sized properly for European plates. I'm not sure when they started building Mustangs for Europe, but I'm pretty sure it was in the last few years.

This article talks about their popularity last year: https://media.ford.com/content/fordmedia/feu/en/news/2018/04...