I used the upgrade install to go from 4.9.1 to 5.0.1.  (I needed the caching performance improvements that went into 5.0.1 because I couldn't add portals due to the large amount of time it was taking to clear the cache when the portal was saved.)

Now I can add portals, but I can't change the settings.  The default language drop down is empty, and changing the portal settings just results in an exception due to trying to set the DefaultLanguage column to NULL in the DB.  If I go to Host->Languages, it just shows a blank screen.  (Incidentally, going to Host->Modules also shows a blank screen, which is odd.)

Any ideas on how I can get it to show a language in that default language drop down?

http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/108/threadid/293762/scope/posts/Default.aspx

I believe I found the problem....

In Library\Services\Localization\Localization.vb there is a method named LoadCultureDropDownList.  This is what populates the drop down list in SiteSettings.ascx when modifying a site's settings.

In 4.9.1's code, the first thing this method did was call GetSupportedLocales.  GetSupportedLocales, upon finding no language packs installed, would create a locale en-US and return it was the list of supported locales.  This method has been marked as obsolete in 5.0.1.

In 5.0.1, it calls GetLocales.  GetLocales does a query from the database and execs GetLanguagesByPortal.  Well the problem is that the 5.0.1 upgrade script would have to insert a language setting in the PortalLanguages table for every portal in the system.  There would also have to be an entry made into this table when a new portal is created.  This is likely not desirable.

Therefore, it looks like either GetLocales or LoadCultureDropDownList needs to detect when no languages are set for a portal, and force en-US as an available in that case.