Your Vanilla Community’s interface can default to one of dozens of languages through our Locales, Settings page
In order to change the default language of your community from US English, you will have to first enable the language you would like to use:
Once you have enabled your language of choice, you may enable it as a default locale in the drop down at the top.
Don’t forget to hit save at the end!
If you’d like to use more than one language, please refer to the Translating Vanilla article