OK – I can’t resist it 
Your language setting of ‘en_US’ is actually 100% correct but is sadly and incorrectly ignored by most software and especially is incorrectly ignored by WP and most WP plugins. And that is also true of this one much to my annoyance.
The first part of a language code represents the parent language. In this case ‘en’ – or the designation for English. If, for the country in question, they ARE the parent of the language then no more code is required.
The second part represents the user country if NOT the parent of the language. In this case ‘US’ – designation of the United States of America.
So – following those rules – the ONLY country that is officially supposed to use a code of ‘en’ is Great Britain.
The USA SHOULD use ‘en_US’ and have a translation file available to translate the parent English into the American spellings.
But – truth is – virtually everyone ignores this so, being English as I am, I always have to put up with the American versions of my own words!