I had the same problem on a DNN 4.3.5. I fixed it by following steps:

1. Looking if files are missing (locales.xml and locales.xml.config)
2. When existing and content is correct, login as host and verify the menus host->languages and admin->languages what is displayed (must be identical with the files)
3. I assume your exception happened when you opened one of these menus above. So you should check the field 'DefaultLanguage' in the table dbo.Portals if this is correct. When not  fix this (i found my error here).

However when this isn't solved, you can debug your system. Set a breakpoint in the file around the line 348 in the file Languages.ascx.vb:

Private Sub BindGrid()
            Dim ds As New DataSet
            Dim dv As DataView

 ->       ds.ReadXml(Server.MapPath(Localization.SupportedLocalesFile))
            dv = ds.Tables(0).DefaultView
            dv.Sort = "name ASC"

            dgLocales.DataSource = dv
            dgLocales.DataKeyField = "key"
            dgLocales.DataBind()
        End Sub

 Looking you'll find data in the objects (ds) why the DataBind fails.

Good luck!

I prefer C# too. The variable dgLocales is only result of a datagrid and dv is the DataView of the DataSet Was really interesting is from where comes this Method BindGrid()?

The points described before didn't help nothing?