It seems that the Javascript "Radzen.getCulture" does not work correctly on iOS and macOS with the Safari Browser. On browsers like Firefox or Chrome it does work as expected.
I've just tested new Blazor application using our sample database with two cultures (en-US and de-DE) and everything worked normally on change from the culture picker and page refresh :
I think the problem is more Blazor related. Safari with German user interface settings will cause server sided Blazor to show the English user interface. Do the same with Firefox on macOS and the German user interface will be shown as expected. It seems that Safari is doing something differently that Blazor is not expecting.