This happened to me in a system installed initially in Spanish. And the following are the steps I followed:
1. Create a new desktop user.
2. At login screen, select the newly created user, change the language options to English, and login.
At this point, the system freezes and you can only see the Trisquel blue background, but the interface doesn't appear.
I had to restart the system. After this, one can login normally with the new user.
I can't reproduce this in a different machine with Trisquel 3.5 final.
I created 4 more users with different languages for each of them and I had to restart the system for 3 of them.
It works for me.