Outlook.com users can change the language to their native speaking language using the options available as part of More mail settings.
Step 1: Login to Outlook.com, click Settings icon and select More mail settings from the list.
Step 2: In the Mail Settings screen, navigate to Customizing Outlook section and click Language option.
Step 3: In the Language screen, choose your preferred language and click the Save button. In the example below, I have scroll down and selected Tamil as the language for my Outlook.com account.
Now your Outlook.com screen should reflect the language change.