Hello
You will need to do this on a computer.
Sign into www.outlook.com. Click on the gear icon on the upper right near your user profile icon to access settings. You might have to click on the three dots first if you don't see the gear icon right away. Then use the screenshot below to guide you to the location you need to change the language back
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On the Mail tab, go to Rules. Are there any rules set up here? If yes, delete them all.
Then check the conditional format, again, if it's there, delete it.
Then, check Forwarding and IMAP, If any items are present, delete them as well.
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If you can't find anything, go to https://account.live.com > Your info > Sign in Preferences. Make sure to check all the aliases here. If you don't recognize any there, delete it.
Then go to Security > Manage How I sign in, make sure all contact info is yours. Then scroll down on the same page, look under App Password, click on remove existing app passwords as well.
Please make sure you change your password and enable two-step verficiation.
Go to your account online, go to Devices, remove any devices you don't recognize.
Then go to Security, click on Sign out everywhere. This is meant to kill any session that's still connected. Unfortunately, this one will take 24 hours to take effect.
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In addition, this can be due to a malicious script running on your device. Are you using Windows device to check emails? Do you use any email app, such as Outlook Classic or New Outlook? Please go to the app, remove the account from settings.
Then go here: https://learn.microsoft.com/defender-endpoint/safety-scanner-download
Download and run the 64 bit scanner, run a full scan. Tell me what the final results are.