To change the default displaying language in Outlook

Step 1: Click the File > Options.
Step 2: In the Outlook Options dialog box, click the Language in the left bar, and go to the Choose Display and Help Languages section, click to select a language from the Display Language box, and then click the Set as Default button below the box.
Step 3: Click the OK button. And then a dialog box comes out to remind you to restart Office, just click the OK button.
Step 4: Restart your Microsoft Outlook.