The first Arabic version of most websites is the English one with dir="rtl" on the page. Columns swap sides, text aligns right, and in a screenshot it looks finished. Arabic readers notice the rest within a few seconds, which is why we treat right-to-left as its own build rather than a setting.
Start with layout written in logical terms. A margin on the left is a margin on the wrong side in Arabic; a margin at the start of the line is correct in both languages. CSS has had logical properties for this for years, margin-inline-start rather than margin-left, and using them from the first stylesheet means the Arabic layout is not a second set of rules waiting to drift out of step with the first.
Then list what must not flip. A logo is a fixed image, so we lock ours to left-to-right. Web addresses, email addresses, phone numbers and code read left to right inside an Arabic sentence, so each one is marked as such, or a number like +1 604 shows its plus sign at the wrong end. Arrows that mean forward should point the other way in Arabic; a clock or a play button should not. We go through every icon and decide.
Typography needs its own values, not borrowed ones. Arabic headlines need more line height than English, or the dots and marks of one line touch the next. Never split an Arabic word into letters for an animation: Arabic letters change shape depending on their neighbours, so separating them breaks the word. Split on spaces instead. And count the font files: every extra weight of a good Arabic family is a download of around 80 KB, which a visitor on a slow mobile connection feels.
Copy length changes the design. The same message can be noticeably longer or shorter in Arabic, so buttons wrap, menus run out of room and a headline that fitted in two lines takes three. We test every page at narrow widths in both languages, because the layout that breaks is usually the one nobody checked in Arabic.
Give each language its own address. A toggle that swaps the language in place cannot be shared or bookmarked, and search engines see one version. Separate URLs, /about and /ar/about, tagged as translations of each other, mean an Arabic link opens in Arabic and both versions can be found. Check third-party widgets too: some booking and chat tools have no Arabic or right-to-left support at all, so the Arabic page needs another way to reach you, such as WhatsApp.
None of this is difficult once it is on a list. What makes an Arabic site feel native is that someone checked each item, in Arabic, before launch, instead of a customer finding them afterwards.