Here’s a useful write-up for Young Sheldon Season 6, Episode 18, titled

Sheldon and Mary are in Germany for Sheldon’s summer study abroad program at Heidelberg University. Sheldon is excited to learn from a renowned physics professor, but quickly becomes disillusioned when the professor insists on teaching history of physics rather than cutting-edge theory. Mary, meanwhile, feels isolated, struggles with the language, and worries about the family back home. A long transatlantic flight (the “10-hour flight” of the title) and cultural clashes dominate their trip.