Just installed your plugin on 1.7.3
I have a dutch/english joomla. So when registering people can choose what language.
When registering wth the plugin active, they get the failure that they didn't choose a language.
Then I switched of the "Encrypt all input fields?" Then I had no problems anymore. I looked in the code, it doesn't notice the language field, so there it goes wrong I think.
Thanks for the work, I had a plugin in old Joomla that was able to ask the user a question, and couldn't find one that could do that, until I found this one.