It is totally true. Look at the literacy rates of countries south of the border, excluding Chile, and you will find 70%+ illiteracy.
We deal with it all the time in California. Our literacy programs are directed at helping them achieve some level of literacy in their original language and then we help them achieve a level of literacy in English so they can function at a higher level in American Society.
Now, go to any report you can find, regarding LDS growth rates and you will find that total numbers are coming from places that have challenges emphasizing competency in their respective languages, with the highest percentage illiteracy outside major cities, which is where you will find LDS success in converts.
I can easily see it and understand exactly why those are the only areas they are getting converts. I do remember reading a few years ago that their retention rate for converts in So America past one year was less than 30%, so they would need to have a lot of numbers to show any growth.