The part of the GUI where input your Arabic text is handled by Qt and that knows about Right-to-Left (RTL) writing systems, such as Arabic. The Text:Simple filter calls MLT’s DynamicText filter, and (I may be wrong but) I suspect that this does not know about RTL.
HTML/CSS does know about RTL, so I would suggest using using the Text:HTML filter instead making sure you use the “dir-‘rtl’” parameter on the HTML tag in question.