I have upgraded from 4.9.5 to 5.2.3, and I'm experiencing some issues.

It is not possible to save Site Settings because the "Default Language" field cannot be null, and the dropdaown is empty. This could be fixed by enabling en-US in the Admin->Languages page. But this page is completely blank. When I try to add the languages module to the Admin->Languages page, nothing happens.

I tried to create a new Language module, using EditLanguages.ascx as a control. With that module I was able to enable other languages (I tried en-UK) besides en-US, but when I tried to enable en-US I got an error saying it is not possible to create duplicate rows in dbo.Languages table (an en-US row was already there, with NULL as fallback language). Also the row for en-UK was there with en-US as fallback.

After doing this it was possible to select en-UK as default language in Admin->Site Settings, but still not en-US.

I have read that there are known issues with languages/localization in DNN 5.2.3, and this may be one of them. I don't need multiple languages in my installation, so I think I should be able to use DNN 5.2.3. What should I do to get it to run with en-US as the default language, and making it possibleto store Site Setting without errors.

Any help appreciated!