If you can't get in directly to the dashboard to revert page or global area versions - sounds like you have tried with your note about /login. You could try a variation:
site.com/index.php/dashboard

The next trick is to create a temporary override for the switch language bock in /application/blocks.
The override would be designed to:
- not require any assets
- not add any header or footer items
- output to the page a simple 'Hello' text.
You should be able to do that by taking the switch language controller.php and view.php and deleting all methods in the controller and replacing all code in the view with: echo 'HELLO'.

I just tried to enter with site.com/index.php/dashboard and it doesn't work. The page takes too much time to load and nothing happens.

As a temporary measure, You might try commenting out the area code in the theme template where you installed the block.

I just tried but I can't access this php page either :(

You will have to access these files via an FTP program or your servers file manager program.