You add Arabic to your app. You put dir="rtl" on the <html> element, reload, and most of the page flips on its own. The navigation moves to the right, the text lines up on the right, and the sidebar swaps sides.
Then you look closer. The back arrow still points left, so it now points forward. The search icon sits on top of the text you type, because its padding is on the wrong side. The carousel slides the wrong way. A price in the order summary has its currency on the wrong end. The heading you styled with a little letter-spacing has fallen apart into disconnected letters.
Setting the direction gets you most of the way. This chapter is about the rest: what flips by itself, what you have to flip, what must never flip, and how Arabic and Hebrew text needs handling differently from Latin text. Much of it is inspired by Ahmad Shadeed's excellent RTL Styling 101, which is worth reading in full alongside this chapter.