There are some issues with Localization in DNN 9, but none we have not been able to fix so far.
1. If you have 2 languages, make sure you have these Portal Aliasses
mysite.com, not default
mysite.com/nl-nl > nl-nl, primary
mysite.com/en-us > en-us, primary
Restart DNN and try again.
2. Try clearing the entries for the page in the TabUrls table
3. Is the DefaultLanguageGuid of the page correct?
It should be equal to the UniqueId of the page in the Default Language (which is EN I guess)
For some reason these sometimes get mixed up on a ML site.
4. Does the parent of a page have the correct "culture" (I guess this is a root page so less likely)
5. When the pages seem correctly translated (when you look at the page settings per language) but the menu only shows the default language, it can be that the language is missing in the Portallocalization table.