Hi @Markku Blikst
The web version (browser-based) of Office 365 is displayed in English, but Outlook and other apps are in Finnish.
Is the browser-based version of Office 365 the "Office online Server" you deployed?
If not, do you mean that the display language of apps for web, such as Word and Excel is English, but for others, such as Outlook for web is Finnish? Besides, what is the display language of Microsoft 365 portal?
Please check whether this thread is helpful to you: Changing the display language of Office Web Applications for a single user. Via this thread, we can get: The language of your Microsoft 365 portal and Microsoft 365 Apps in hybrid environments is controlled by the preferredLanguage parameter in local AD.
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