As far as I'm concern, I've met a few who does speak foreign languages fluently,but not be able to write or read in that particular languages just because they have friends who does speak that particular languages.This is called,social speakers.They learn to communicate in that particular languages just for the sake of communication skills.They learn by listening,watching TV dramas or by communicating with locals.This usually happen when if you are staying/living in a country which does not speak your language neither do they speak in language that are well known to you(ie :English).
On the other hand,some people have difficulties in reading and writing.They are said to have dyslexia.Dyslexia is characterized by difficulty with learning to read fluently and with accurate comprehension despite normal intelligence.This includes difficulty with phonological awareness, phonological decoding, processing speed, orthographic coding, auditory short-term memory, language skills/verbal comprehension, and/or rapid naming.
Developmental reading disorder (DRD) is the most common learning disability. Dyslexia is the most recognized of reading disorders, however not all reading disorders are linked to dyslexia.
Some see dyslexia as distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or poor or inadequate reading instruction.There are three proposed cognitive subtypes of dyslexia (auditory, visual and attentional), although individual cases of dyslexia are better explained by specific underlying neuropsychological deficits and co-occurring learning disabilities (e.g. attention-deficit/hyperactivity disorder, math disability, etc.). Although it is considered to be a receptive language-based learning disability in the research literature, dyslexia also affects one's expressive language skills.