Force user to write in specific language (Validation)

If this is server-side validation to detect the language of a post, then you could use a library like the ones discussed in these threads to detect the language and to return an error if the language is not Arabic:

• https://stackoverflow.com/questions/1441562/detect-language-from-string-in-php

Users would still be able to type non-Arabic text into the field on the client side, but it would prevent, for example, a user from posting comments in Japanese onto an Arabic website.

There could also be a character whitelist on the client and/or server-side, implemented similarly to what is discussed here:

• https://stackoverflow.com/questions/11517440/allow-only-arabic-letters-in-textbox-using-javascript