Customers have been reporting issues with the display language of Mail & Calendar app following Windows updates. The KB article below contains information on the issue and provides a workaround for users.
https://support.office.com/article/display-language-problems-in-mail-and-calendar-apps-in-windows-10...
Note: If you notice any other display language issues in the Mail app or Calendar app, please send us your feedback.
Thanks,
Declan
