The problem you describe is common to all learners of foreign languages, not just to Arab students. Arab learners have an additional problem in that they are dealing with an alphabet that is different to their own. (You don't say which country you are in or whether the students are familiar with other alphabets. so I can't comment on this area).
If your students are not writing freely in English, this could mean they're not thinking in English. They're still thinking in their mother tongue which is only natural and in no way indicative of them being "mentally squint". I don't agree with you when you say this. You don't say what level the students are nor how long they've been learning English. You need to remember that it takes students a long time to start thinking in a foreign language.
To encourage students to think in English, a teacher needs to speak English at all times and encourage students to do the same. Try to avoid translation whenever you can. When teaching vocabulary, you can give an explanation in English instead of translating. This can take a lot longer than translation, so you can translate at times for the sake of speed. You can also try a process approach to writing, getting students to go through each stage in the target language. For ideas about process writing, you can see my answers to Omar on the topic.
I can't give your more specific information than this without knowing the level of the students. You can write in with more questions if you like.