WordPress Language translation, stuff i forget.

functions.php

*/
https://localise.biz/help/wordpress/loading-translations
"If WordPress fails to find a MO file at this exact location, it will then look in the global languages directory"

So in short, WordPress will look in only two places for a theme's MO file:

{absolute_path}/{locale}.mo
wp-content/languages/themes/{domain}-{locale}.mo

e.g. for twentyfourteen's French translations:

wp-content/themes/twentyfourteen/languages/fr_FR.mo
wp-content/languages/themes/twentyfourteen-fr_FR.mo

/*

add_filter('locale', 'different_backend_lang', 1);
function different_backend_lang($locale) {
    if ( is_admin() ) {
        $locale = 'en_US'; // Backend
    } else
        $locale = 'da_DK'; // Frontend

    return $locale;
}