Learning a new language is difficult enough if you know the letters: for example, Turkish to English, Polish to English.  If your mother tongue uses different letters (e.g. Bulgarian, Persian, or Japanese to English) a learner must learn a new set of symbols, how they link together, CAPITALS/small letters, etc.

This is particularly important for visual learners who are used to seeing and reading  words.   If teachers forget this difficulty, students may not get enough time to decode written words and can then be seen as failing.  It is important to understand what skills are needed to do a task: are these skills being assessed, or just 'grammar' or 'vocabulary'? 

My Persian teacher was disappointed when he gave me my exam paper: "You only got 60".  "No, I got 60!" I replied, because I was reading like a 6 year old, concentrating on each letter to get the meaning.  Others in the class were fluent readers of Arabic letters but as our listening and speaking abilities were similar the teacher expected us to get the same grades in the exam.