I’ve been seeing the use of the phrase “functinally illiterate”, and it got me thinking that I don’t think there is such a thing when it’s speech instead of the written word. There are plenty of instances I can think of where I or someone I knew was simply not capable of or didn’t want to try to understand something that was said. Is there something that means not bring able or refusing to comprehend speech when that person is fluent in the language being spoken?
Not capable at all would be non-verbal. This includes vocal and non-vocal verbal communication such as PECs and sign. Non-vocal is not speaking out loud. Able to speak but not doing so may be called selectively verbal.
Not trying at all could have several outcomes and reasons behavioral and medical and would not be any of these. They may not learn to read so lack literacy or may not learn properly how to speak and lack fluency in their first language.
Edit: upon reading some of your other comments, you may be thinking of delays in receptive and expressive language. Which can have many causes.