Change localised version 3.9.1 back to us_en version

Hi all
I′m a newbie in wordpress, put I been trying to solve this problem on my own for more than a week now. I really need to ask for help:

When I updated to 3.9.1 I was not aware, that it was called 3.9.1 da_DK, and now my admin language is danish.
I need it to be default en_US for several reasons.

No matter what I write in wplang it′s still in danish. Right now it looks like:
define(‘WPLANG’, ”);
And I′m unable to locate any .mo files in the language folder ( using twenty thirteen theme).

As mentioned: I′m now out of options due to my lack of experience.

If my only option is to reinstall an en_US version, would anyone be so kind to provide me with a link to the right version and a link to a “step by step” guide for newbies?

Best
Zepli