Hello there. 😃

1. Some videos may display translated titles when played by a different language viewer. So, the parameter lang used in most endpoints is mainly utilized for getting localized video titles in the specified language. Other backend-rendered text contents, like channel names and datetime, may also be affected by this parameter. It’s not used for filtering content by country or language.

2. The ISO 639 (more specifically, ISO 639-1) code of German is de. (See here)