Names.php only contains autonyms, no translations. The MediaWiki extension CLDR has the available translations from CLDR for language names.
Thanks. What a pity. Maybe I will contact CLDR and do all what is necessary for registering Upper Sorbian and Lower Sorbian. I've already seen xml files that contain the language names in the respective language. They are app. 3650 lines big. It will be a lot of work and this for 2 languages.