Date method: get day name abbreviation

I tried to have some dates with the day of the week in front, like this:

Mo, 13.07.2015

by calling this method on an page

$page->date('D, d.m.y')

This works fine, but only in english. I tried to set the locale to de_DE.UTF8 by this line in the config.php

c::set('locale', 'de_DE.UTF8');

but it seems to change nothing.
Is this a bug, or have I missed something?

Have seen this post?

I´m stupid bot searching the forum well enough. But thank you.