Change the language or switch to dark mode

To change the language, click your name at the bottom of the left menu. In the menu that opens, the Language row shows the current language on a small button.
Click that button and pick Deutsch, English or 中文. The page reloads in the new language.
If the page holds typing you have not saved, the app warns that it will be lost. Click Switch anyway to continue, or Cancel to stay.
The language is remembered for your account, and the mails you receive follow it too.
For light or dark mode, open the same menu and click Settings. Choose the tab Theme.
Under Appearance click Light, Dark or System. System follows your computer or phone.