Currently Airmail uses default date format from system settings. Btw, even if my system locale is non-english i'm usually sending email in english, not all of my recipients speaks my language. The problem that when I reply to an email, Airmail puts a date using system locale into a quote of a previous email.
Like: "On 8 декабря 2014 г. at 4:02:15, *** (**@gmail.com) wrote:"
I'd like to have something like "On 8 Dec 2014 at 4:02:15, ** (***@gmail.com) wrote:" instead. You know, because not everybody in the world understand what "декабря" means.
There is no way to configure date format, and app uses system format even if I choose English locale for app (so it works for menus/ui only, and ignored by email composer)