Is this a bug report?

No

Have you read the Contributing Guidelines on issues?

Yes

Steps to Reproduce

1. Clone https://github.com/reasonml/reasonml.github.io.
2. Change website/languages.js to just have Deutsch as the only enabled: true.
3. cd website and yarn start.
4. http://localhost:3000/ still appears in English.
5. Navigate to http://localhost:3000/de and page is broken, see image below:

Consider adding ability to change the default language from English to something else in Docusaurus.