Clarify that $wgAmericanDates is not limited to English

$wgAmericanDates is used whenever the defaultDateFormat is set to
"dmy or mdy", which is not only true for English, but currently also
(among others) for Macedonian, or Persian.

Change-Id: I75fd239bc617e4fc2caa738923021b9d22adb1a1
Merge "Clarify that $wgAmericanDates is not limited to English"