Hi, AndreasBas
After consulting with seniors from our team, based on the tests and checking for all the setting in Outlook.
There's no such a setting which can change the language of Outlook search bar keywords.
Therefore, since it is approaching into the design-level, please be kindly to suggest as a new feature to Microsoft Outlook's engineering team in here - Microsoft Outlook Feedback.
Thanks for your time and your understanding would be highly appreciated.
Sincerely,
Alex | Microsoft Community Moderator.