I just checked it with Firefox where I have "de" as first language and the page loaded with DE as default. In Vivaldi I have "en" and it loaded EN as default. Only thing I would change is the setlang .. make it russischprofi.de/en/kontakt and add all variations to header (Google will love you )
if you want to set default language independent of user preferred language, use this: define('I18N_IGNORE_USER_LANGUAGE',true);
if you want to set default language independent of user preferred language, use this: define('I18N_IGNORE_USER_LANGUAGE',true);