I had the same problem and noticed that the DT-Register language is identical to the default Joomla language at the time of installing DT-Register.
When you want to change it afterwards, I did it as follows:
- set the default site and backend language for Joomla at the desired language
- reinstall DT-Register (existing data won't get lost)
- now DT-Register has the same language as the default Joomla language
- If you want, you can now change the default Joomla language but DT-Register will remain the original installed language.
Don't know if there is an easier way but this worked for me...
Johan