If you are writing Arabic text in the forum, then you can enclose this text in special tags which will render the text in a better and bigger font. It also takes automatic care of right-to-left markup.
The tag for arabic is ar:
Example with tags: السلام عليكم، أتكلم اللغة العربية
Example without tags: السلام عليكم، أتكلم اللغة العربية
Vocalised example with tags:كَتَبَ الْرَجُولُ الْكِتابَ
Vocalised example without tags: كَتَبَ الْرَجُولُ الْكِتابَ
In the post editor, you will find buttons for these tags.