To change the default language for a Google Group, follow these steps:
Enter the MANAGE section of the group
Navigate to the Information > General Information settings
Finally, select your new group language from the drop-down menu
As always, remember to select SAVE when you make changes
NOTE: Changing the language setting will change the language for ALL members of the group; this is not a change specific to your account.
Posted by K. Stesin'13 7/5/12