The language code list at [lang section](https://www.mkdocs.org/user-guide/configuration/#lang) contains two errors. `du: Dutch` should be `nl: Dutch` and `jp: Japanese` should be `ja: Japanese`. ref. https://www.loc.gov/standards/iso639-2/php/English_list.php
The language code list at lang section contains two errors.
du: Dutchshould benl: Dutchandjp: Japaneseshould beja: Japanese.ref. https://www.loc.gov/standards/iso639-2/php/English_list.php