Is French just the most commonly spoken common language, even in Germany and Czechia?
No. This title is likely taken by Turkish.
Or is it something else?
Many phrases from European languages are common knowledge across Europe. I’m about to go grab some coffee. When I walk in to the coffee shop, I’m free to say “Hello” in one of 10+ languages and no one will think anything of it. Why would I do that? Maybe because I’m in the mood. Ciao!