I've created a modern Sharepoint site with 2 languages English and Arabic, the navigation menu dose not change to the selected language when I chose the other language, however all site content changed to the selected language.
Julie_MSFT's advice is the simplest way to localize the navigation, but it only works for modern sites where the Multilingual Page Publishing feature is on. For other types of sites you have to edit the navigation text while your language is successfully set to that language. That means you edit the Arabic navigation text while your language is Arabic (page layout will be right to left) and edit the English navigation text while the language is English. The URL does not vary with language, but the text can.