Sending, receiving, and organizing email in Outlook.com
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If you received a suspicious activity alert and things on the account started changing, the first priority is securing the account.
Were you able to sign in and change the password already? Please test https://account.microsoft.com
If not, try securing the account immediately through the Microsoft account security page and review recent sign-in activity to see if there were any unfamiliar logins.
The language issue could be because someone changed the account region, browser settings, or specific Microsoft service settings. Even if the account's preferred language still shows English, other settings may have been modified.