How to change the default forum language
From Online Manual
Choosing a Default Language from Language Settings
- Go to Admin > Configuration > Languages > Settings.
- Find "Default Forum Language" and select the language you want for your forum.
- Select the Save button.
Getting Different Languages for Your Forum -- Add Language
- Go to Admin > Configuration > Languages > Add Language.
- Type the name of a language into the input box.
- Select the search button.
- The page will be updated with a list of all matching language packs from simplemachines.org.
- Use the "Install" link to the far right of your desired language to install the language pack for your forum.
Language packs may also be found at Language Site Downloads.
For more information, see Languages