The holiday calendar should have the option to display the events in the local language. The English translation is often a bit weird and it is therefore hard to grasp at a glance the purpose and reason for the holiday.

Furthermore, It should be possible to decouple this option from the UI language. As someone who speaks multiple languages, it is always a vastly better experience to use English for the user interface. Compared to other languages, English is more searchable, consistent across applications, and has more thought and care put into it.