Open Outlook
Go to File
Select Options
Click Language
Under Office display language, choose the language you want
Under Office authoring languages and proofing, choose the language you want
Click Set as Preferred if available
Click OK
Restart Outlook
To change the Outlook web language, open Outlook on the web
Click Settings
Click View all Outlook settings
Go to General
Select Language and time
Choose your language
Click Save
To change the Outlook mobile app language, change your device language in system settings
Restart the Outlook app