Mailbox.org frontpage ignores browser language setting

When I need to log in using the Mailbox.org frontpage (https://mailbox.org/), the website and login dialogs are all presented in German.
There is an English version available, however it is not selected by default, nor is the setting persistently stored.

My browser language settings are Dutch and English (no German configured).

Expected behavior is: use English when German is not a configured browser language and persistently store the chosen language for the user.