With *new* Language Settings for Groups, group creators and admins can now choose the language for particular groups on the "Edit Group" page, and all parts of the group's interface will appear in that language.
When participating in a group with its language set to French, for example, phrases like "Started by" or action links like "Favorite" will be translated to "Créé par" and "Favori." Even better: All email notifications received from participating in the group will now be in French, too.
This is perfect for language-orientated groups!
All Group members were previously were forced to use an English interface. With this new option, they'll have a much more native and comfortable experience within the group.
Group owners, as outlined above you can choose your language for your group by clicing on "Edit Group" on your Group page.