You can restrict backend users and groups to edit records only in specific languages. The permissions are enforced over all tables by checking the language defined in the "languageField" of the table. The "Limit to languages" setting is found in the "Access Lists" tab:
Settings from groups and sub-groups are all added together.
If a user has no "Limit to languages" checkboxes set at all he can edit all languages .
Regardless of "Limit to languages" all users can edit records with "[All]" language.
Setting "Limit to languages" is not only good to protect translators from editing other languages or default content - it is also helpful to limit the confusion a user might have when faced with modules displaying translations for a host of other languages!