Restrict language based content editing for nodes and blocks
For your solution you can implement hook_node_access, as you see this function takes 3 arguments $node which is the content the user wants to make an operation on it, there you can check content's language, $op is the operation which user wants to make on the content which in your case you can check if it is edition, and finally $account takes the information about the user so you can decide if user is allowed to do the operation or not.

One big consideration is that this hook is not fired for the super user (user with uid = 1) as he bypasses almost all permissions, so in this case you can use hook_init and make a redirect based on node arguments.