One of my favorite observations about forms that are in Arabic and English (or any left-to-right and right-to-left language pair) is that you don't need to pick one language as the primary language. The trick is to put the English on the left, a blank field in the middle, and the Arabic on the right. This way an English speaker will naturally read the form as an English form with the Arabic translation off to the rig…
Your text inputs still need to flow either left-to-right or right-to-left though? In which use case would this be useful?
https://www.w3schools.com/tags/att_global_dir.asp
https://caniuse.com/mdn-html_global_attributes_dir
Edit: oh use case? Imagine a PDF form you'd like someone to fill out, or a page that may get printed. You could create two forms and let the person filling out the form pick their language, but the person processing the form may prefer to read the form in the other language.