I'm looking for a simple method to disallow edition at the top level of a multilingual web site, so only content of language folders can be modified (by non-admin users).
By changing the Modify Portal Content permission for the related content to Manager only?
Solution 1
Do not assign global roles.
Put all editors users in a group, assign the group to all language root folders (LRF) with the local roles needed.
Solution 2
This works with global roles:
Add a rolemap.xml to your profile where role contributor/ editor do not get any default add/edit/... permissions.
Create a workflow assigned to the LRF where editors regain those permissions.