E.g., application.systemLocale returns "en_US", while V8 functions like Intl.NumberFormat want to see "en-US".
Annoying. Any good reason?
No good reason. I think both are considered correct. If there are a lot of people preferring one to the other, I will raise this as a feature request.
Fair 'nuff. I guess we can just deal. 
(Whinge, whinge.)
Personally, while I see your point, @cpryland, I still prefer the underscore version as it allows for things like myStrings.en_US.aString (constructed example), which enables syntax checking (using ['en_US'] usually doesn’t check if it exists). I think (from my research when looking at this topic) that that’s also the reason the underscore is considered standard in the Java and POSIX world.
I totally get the other side of the argument, but as @stevekwak asked for opinions, I’d say I prefer the underscore version…