Honestly, YANBU.
For everyone saying she got caught up in "hosting." Hosting who exactly? Part of hosting is making sure that people feel included and not left out. I don't necessarily think the friend is in the wrong here or a bad friend-potentially a poor host if she happily allowed a friend who knew no one and didn't speak the language to sit alone for 2 hours-but I doubt it was intentional.
However, I can fully appreciate where OP is coming from. I have social anxiety too, and would really struggle to simply get up and join a table full of people that I don't know and don't fully speak the language. If you don't have SA, you don't really know what it's like and for everyone saying "you're an adult" yes, she is, but if it was that simple, SA wouldn't exist, would it?
Yes, OP's anxiety is not anyone else's responsibility, but one would make an assumption and not an unfair one at that-that if a friend invites you to an event where you don't know anyone-they would make an effort to try and introduce you and involve you somehow. That doesn't mean that OP expected her friend to be by her side all day, acting as translator.
It's pretty basic hosting skills, IMO and failing to do that, means she failed as a hostess. I wouldn't necessarily raise it, OP, but I understand where you're coming from completely and just maybe don't attend events with her if you don't know anyone else or work on your SA if you can to build your confidence.
It's not UR to expect a host to introduce their guests and anyone saying otherwise is a poor host.