Iris,
You are not alone.I too am a corrections educator. You described a 
similar situation to what I see in my job in VT, a simply huge variety 
of abilities and levels. Your situation sounds a bit more complicated 
than mine because the vast majority of my students are able to speak 
English.

I think that the hardest cases for me are when traumatic brain injuries 
co-mingle with chronic drug and alcohol abuse. It is so unpredictable 
what kinds of learning problems these individuals may experience.

It sounds like you are extremely frustrated with your work with the 
offenders. If you are given freedom to develop the education program 
where you work, you might look at it as a wonderful opportunity to 
create a system that works for your guys where nothing has ever worked 
before.

Ten things I've learned as I helped build our high school, the Community 
High School of Vermont, are:
1. You can't help everybody.
2. Everyone comes to you at his own level and works at his own pace.
3. Some problems of individuals are bigger than education.
4. It is best to have no expectations for your students, not low 
expectations. No expectations.
5. Appreciate whatever gains might be made, no matter what they may  be.
6. Every student is good at something.
7. Students have to direct the course of their education. Teachers 
assist them.
8. Don't try doing it the other way around or you will fail miserably 
and drive all your students away.
9. You may not know when you will gain a new student or lose an old 
student so your classes have to be self-standing, not dependent on a 
sequence of classes the student may have missed.
10. These things motivate learners. and a motivated learner cannot be 
stopped. He will learn more than you ever thought possible, learning and 
emotional disabilities become irrelevant.

Best wishes,
Tom Woods