I know I can block communities but can I block entire federations? I wanted to block foreign language ones which didn’t mark their posts with the correct language etc. so I wouldn’t see the content.
Not without running your own instance. As a user you can block users or communities. To muck about with federation and block a whole instance, you need to be an instance admin. Note that unlike community blocking, this will block ALL interaction with users on that instance. You won’t see any of their posts or comments to any community again.