SharePoint online communication site navigation not translating into different language

The navigation items will show the language versions based on the visiting users' preferred language. The language toggle you see in the header section will only change the page contents, not the site-wide informational sections (e.g. Site Name, Navigation & Footer)

As an example, if I had Spanish set as my preferred language in my Microsoft 365 profile and Spain had been added to the navigation items of the site I was visiting, I would automatically be shown the Spanish versions of those navigation items. The default language of a site is set at the point of creation and cannot be changed. If my preferred language is not available, it will then default to the default language of the site (e.g. If I had French set as my preferred language but only English and Spanish were available on the site, I would be presented the default English version, as that is the sites primary language.

There's some additional information in this article here:

https://support.microsoft.com/en-gb/office/create-multilingual-sharepoint-sites-pages-and-news-2bb7d610-5453-41c6-a0e8-6f40b3ed750c#bkmk_muitranslations