Pretty sure you add / change and set defaults on the sites page for this community.
When you add a language you can then rearrange their order and set as default.
Here:
- Click Site Configuration | Languages on the Overview tab.
- Click Add Languages.
- Select the languages you want to add to your site.
- If necessary, reorder the list as you want it to appear in any language selector.
- Save your changes.
To set default:
- Click Site Configuration | Languages on the Overview tab.
- Select a language in the Default Site Language drop-down list.
Reference here