We are trying to recreate a custom multilingual experience for a company and we have explored many different ideas.
Please note that the main issue is the site language, not its content (as we can control that pretty easily as we're using SPFx and exclusively custom WebParts)
1 - One site (to rule them all), supporting the necessary different languages, with a picker to change the user preferred language in his profile.
Main downside: up to 5 minutes for the site to display in the new correct language.
2 - One site collection with a redirection to the correct subsite according to the user preference + a "language" picker that redirects to the desired subsite. Each subsite supports only one language.
Main downside: it is not possible to have a communication site that is not a rootweb, so its appearance is not perfect (and global styles take time to be applied on page loading, so there's a glitch)
3 - One site collection per language and one to redirect to the correct one.
Main downside: it is not possible to create a communication site of the desired language (Official Microsoft Documentation)
As Microsoft seems to want to kill subwebs (direct communication and features rollout of hubs) and have a site only structure, we cannot comfortably go towards solution number 2.
Concerning the third option, as we can only create communication sites in french, and it does not depend on the users preferred language (tenant AND sharepoint), I believe, as Someone inferred, that it depends on the main tenant language, but I cannot use that as a solution to create a multilingual site.
Do you guys have any insights ?