After several complaints about this, I tried opening Special:CX in Internet Explorer 9. I saw a message saying "Could not connect to the server".
At the same time, it did work with Firefox. Also, we officially support IE 10 and up.
The message should be: "Content Translation cannot be used in Internet Explorer 9".
Tagging good first task, because we already have a browser support matrix in the code, and it's probably just a matter of adding a relevant translatable message and a condition in the right place.