SharePoint posts is very limited in supporting Arabic and other Right-to-Left languages. You can't format paragraph as RTL or LTR. you should distinguish between Right-to-Left and Right aligned paragraph, they are totally different thing. This missing feature prevented our company from using lot of SharePoint features.

Some of this concept is also missing or mis-behaving in Microsoft Teams:

• - When you press Ctrl-Right Shift to make the paragraph RTL and (right aligned at the same time) this works fine, but after pressing Enter to start a new paragraph or a new post the new paragraph stay RTL but switches to Left aligned.
• - Formatting RTL bulleted paragraphs is not correct, the bullet placed at the right side of the edit box and the text is placed at the right side of the text.
• - When I want to insert Code in RTL paragraph (the paragraph is right aligned) the code also displayed as right aligned. the correct behaviour is to be displayed as left aligned (code should always be displayed as left aligned and LTR).

I can provide some images and more explanation anytime.