CrazyCore is just ignoring changes to the config file.

Before:

defaultLanguage: nl_nl
preloadedLanguages:
- nl_nl
- en_en

When manually edited:

defaultLanguage: en_en
preloadedLanguages:
- en_en

After reloading:

defaultLanguage: nl_nl
preloadedLanguages:
- nl_nl
- en_en

Urgent.

Have you tried:
a) Using /crazycore reload *
b) Stopping the server and then applying the changes?