Is there a way to detect if the user is running the AIR application under en_GB locale on Windows? Capabilities.language returns only "en" and Capabilities.languages[0] returns "en_US" :(
Unfortunately, no. But it will be something soon (sorry, can't tell you more now)!
Check here: http://www.adobe.com/cfusion/event/index.cfm?event=detail&id=1489921
"Get the inside scoop on the new mobile features in Flash Player 10.1, as well as the new global error handling, UI, globalization, and media playback features."
Now the globalization features are out in Flash Player 10.1 you can use them. Check out the documentation for them here: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/globalization/package-detail.html
and more info here: http://www.adobe.com/devnet/flashplayer/articles/flash_globalization_package.html#articlecontentAdobe_numberedheader
You can easily get the default local as a string like so:
new StringTools(LocaleID.DEFAULT).actualLocaleIDName; // returns en-GB if region is United Kingdom on OSX