Classic sites had lots of language-specific text in their templates. That meant that the MUI never quite translated the entire interface, because a lot of strings were hardcoded in the template. But modern sites have a lot less that is language-specific.
For example, if you create a new modern site it will have a sample Hero webpart in it, which comes with a lot of sample text like "Welcome! Click Edit at the top right of the page to start customizing" or "Make an impression", "Share your strategy", and so on. Now change your profile language and reload the same page. If you haven't changed the Hero webpart yet, then entire text of the Hero webpart will switch to the new language. The Hero webpart template is not language-specific. This is discussed in How Much Multilingual Support Do SharePoint Online Communication Sites Have?
The same thing happens for most out-of-the-box modern webparts, apart from the odd bug which will be cleaned up eventually. Similarly for dates: date formatting in modern webparts that have dates by and large follows the language of the user, unlike dates in classic webparts.
The only major difference when you create a modern site in a different language is that the URLs and internal names of standard lists an libraries, like "Shared Documents" and "Site Assets" will be created in the language of the site. The title that is visible to users will change according to the user's language, so it will always be called "Site Assets" in English, while it is called "Éléments du site" in French, that is just the MUI doing its bit, but the name that is visible to programmers will be different. That is more of a hindrance than a help, programmers have to deal with the fact that the list/library URLs are not fixed.
So what do you gain when you use this new SharePoint Online functionality? Not much, really. If you're a programmer it is helpful to test what happens to your customizations and apps if someone installs them on a tenant whose default language is different from your own, but for normal use, it doesn't change anything.
What would be nice would be if developers would routinely follow guidance about making custom webparts and customizations language-independent. It is possible to localize SPFx webparts if you really set your mind to it, but it requires extra effort since the process is quite involved. It's not as simple as using .resx files, but the principle is the same. Then it won't matter what is the language of the site, the entire UI, including the UI of the customizations, will be language-independent.