Bug 16887: Update intl.accept_languages value
We previously spoofed the `intl.accept_languages` value to "en-us, en" but with the update to ESR 38 Firefox does not like the lower case country code anymore. See: https://html5.org/r/8458 and https://bugzilla.mozilla.org/show_bug.cgi?id=889335 for the reasons behind that. We adapt our code accordingly and set the default value to "en-US, en" now. This fixes #13277 as well. Thanks for this patch, cypherpunk.