How can I determine the current locale, or at least the currency symbol?

After some research, it looks like this is a problem with the Mono version used in Unity - current locale is always returned as "en-US". As a workaround, if your application will work under Windows only, you can use PInvoke (example found here):

 [System.Runtime.InteropServices.DllImport("KERNEL32.DLL")]
 private static extern int GetSystemDefaultLCID();

And then, you can get current culture by calling:

 var currentCulture = new CultureInfo(GetSystemDefaultLCID());

You can also set current culture globally, by using:

 Thread.CurrentThread.CurrentCulture = new CultureInfo(GetSystemDefaultLCID());
 Thread.CurrentThread.CurrentUICulture = new CultureInfo(GetSystemDefaultLCID());

at the beginning of your application. After this, CultureInfo.CurrentCulture (or CurrentUICulture) will return proper info, and {0:C} format should work as expected.

Unfortunately, RegionInfo.CurrentRegion will not return your true RegionInfo. I guess that Unity accesses CurrentRegion property before any of your scripts, and by accessing it, sets it to the earlier culture, which is en-US. But I believe this is not a big issue, as you can always create new RegionInfo(lcid), in case you need it.

One issue which doesn't work - if someone has some specific regional settings in Windows, e.g. changed $ to $$, the currency symbol returned will be $. At least it's the behavior on my machine.