I believe that adult literacy improvement has many similarities with foreign language training. In both cases a person needs to train a skill through constant practice. It is not a matter of intelligence or theoretical knowledge, so much as it is a matter of developing certain habits.I believe that The Linguist can be an effective tool for both.
Both the ESL learner and the adult literacy learner need to make sense of language input that is at first strange or difficult for them. Vocabulary acquisition and constant practice are key. In both cases it is essential to encounter new words and phrases often in different contexts so that they start to become familiar. In both cases the combination of audio and written text create a more powerul learning environment than written text alone. In both cases the accumulation of this vocabulary is more important than the study of rules of grammar.
If the learner can choose content of interest, more or less at his or her level of difficulty the task can be enjoyable and not frustrating. If the task is enjoyable the learner will continue. Continued effort will bring success, but the whole process should be enjoyable and satisfying. Unfortunately at present it is often boring, and even humiliating.