[Theme Horse Support Team]

Hi Urs,

You just need to add this .po and .mo file inside language folder which is inside theme folder -> library -> languages and add your file. Doing this while updating to new version your new added language file will be lost and you need to add it again and again. So you better make a child theme and add your language file.

And goto the root of the wordpress there is wp-config.php file. open it and add this following code

define ('WPLANG', 'de_DE');

Hope this may help you
Thank you!