When you create a new Google Account, Google uses your location to set up the default language for Gmail, YouTube and its other services.
It can happen that you created your Gmail account being in a foreign area and due to this fact you have its language by default. Here is how to switch to your native or preferable language in Gmail web interface.
Click the gear icon in the upper right corner and go to “Settings”:
The General tab will open and you will see “Gmail display language” setting:
Select the preferable language. For example, we would like to change to German. So we select Deutsch in the drop-down menu:
Do not forget to click “Save Changes”:
Result: the web interface is now in German:
Now you can enjoy using Gmail in your native or desired language. You can always switch to another one or restore the settings you had before.