[andreashansen]

Xamarin would probably be the framework/tool of choice if you want C#. Other than that, most (popular) cross-platform app development frameworks I've found do use JavaScript. "Disclaimer": I'm in the middle of writing a thesis on the subject.

[mwcampbell]

Since you're writing a thesis on cross-platform frameworks, have you looked at the toolchain that Google used for Inbox? They write their cross-platform non-UI code in Java. On Android, that's native. For iOS, they compile it to Objective-C using a tool they wrote called J2ObjC. For the browser app, they use GWT. The UI code is platform-specific and written in each platform's native language, i.e. Java, ObjC, and JS. They don't even try for cross-platform UI code, as Ray Cromwell explained here:

https://news.ycombinator.com/item?id=8965349

This toolchain doesn't cover UWP, except indirectly via UWP's support for web apps.