For the verbally challenged, I would suggest trying an existing product to see how much success you have. Ignore Google cloud speech at the moment because It has features that may cause problems. I would think that an offline product such as Android speech to text or Dragon Naturally Speaking would be a good inexpensive choice. If you are successful in getting the speech to text working, then it shouldn't take to much work to allow it to go speech to text to speech.
In laymans terms, you are most interested in the pattern recognition of these products. You will need to approach this differently than the standard speech recognition training. First, I recommend asking the product developers if they have an empty dictionary to eliminate false matches. You should ask for any recommendations and options that relate to your situation. Also ask for any hints, tricks and help they will offer for adding vocabulary and training the recognition software.
Start with a few words to see if you can get any success with it. Don't read from a book because it may cause the speech pattern to change. It's best to start with situations where you feel are the most consistent with good patterns.
Very important is not to get excited about this while training the software. First, this will cause you to help the product work instead of properly training it to recognize normal patterns. Second, the verbally challenged may feed from your excitement but that can quickly change to frustration or fatigue. If you remain enthused without being excited, you will help them work at a pace they can sustain and hopefully get them to speak normally rather than using exagerated speech. Don't make this a game where success is the translation. In fact, you will have the best success if this is trained while doing a normal activity rather than focussing on the training.
To start, you will need to add some vocabulary and train the software to recognize those words. This will take a considerable amount of work so don't expect this to be the advertised 15 minute training. For the verbally challenged, situations have a great impact on their speech pattern. Something as simple as reading can have a major affect because a different part of the brain is involved. Familiarty, stress, excitement and boredom are just some of the factors you will need to consider whether you need dictionaries for specific situations or environments. Or maybe you can train it to allow for different situations.
Good luck in your endeavour.