Just did a fresh install, 4 times, of DNN 5.1.2 and any update to the admin | site settings page results in an error. Whether accessing as host or site admin, the Admin | Site Settings | Advanced Settings | Default Language dropdown box is empty and pulling down to select does not reveal anything.

This is the text of the error:

Error: is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: Cannot insert the value NULL into column 'DefaultLanguage', table 'C:\INETPUB\WWWROOT\APP_DATA\DATABASE.MDF.dbo.Portals'; column does not allow nulls. UPDATE fails. The statement has been terminated. ---> System.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'DefaultLanguage', table 'C:\INETPUB\WWWROOT\APP_DATA\DATABASE.MDF.dbo.Portals'; column does not allow nulls. UPDATE fails. The statement has been terminated. at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString) at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async) at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe) at System.Data.SqlClient.SqlCommand.ExecuteNonQuery() at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteNonQuery(SqlConnection connection, CommandType commandType, String commandText, SqlParameter[] commandParameters) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteNonQuery(String connectionString, CommandType commandType, String commandText, SqlParameter[] commandParameters) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteNonQuery(String connectionString, String spName, Object[] parameterValues) at DotNetNuke.Data.SqlDataProvider.UpdatePortalInfo(Int32 PortalId, String PortalName, String LogoFile, String FooterText, DateTime ExpiryDate, Int32 UserRegistration, Int32 BannerAdvertising, String Currency, Int32 AdministratorId, Double HostFee, Double HostSpace, Int32 PageQuota, Int32 UserQuota, String PaymentProcessor, String ProcessorUserId, String ProcessorPassword, String Description, String KeyWords, String BackgroundFile, Int32 SiteLogHistory, Int32 SplashTabId, Int32 HomeTabId, Int32 LoginTabId, Int32 UserTabId, String DefaultLanguage, Int32 TimeZoneOffset, String HomeDirectory, Int32 lastModifiedByUserID) at DotNetNuke.Entities.Portals.PortalController.UpdatePortalInfo(Int32 PortalId, String PortalName, String LogoFile, String FooterText, DateTime ExpiryDate, Int32 UserRegistration, Int32 BannerAdvertising, String Currency, Int32 AdministratorId, Double HostFee, Double HostSpace, Int32 PageQuota, Int32 UserQuota, String PaymentProcessor, String ProcessorUserId, String ProcessorPassword, String Description, String KeyWords, String BackgroundFile, Int32 SiteLogHistory, Int32 SplashTabId, Int32 HomeTabId, Int32 LoginTabId, Int32 UserTabId, String DefaultLanguage, Int32 TimeZoneOffset, String HomeDirectory) at DotNetNuke.Modules.Admin.Portals.SiteSettings.cmdUpdate_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

Found a work-around by going in to Admin > Languages and editing the Language to English (United States)

Threw me for a loop for sure. Like said, tried wiping and re-installing 4 times and things worked fine until I went in to the Admin > Site Settings menu. Changing anything like logo, copyright text, skin, etc. then clicking "update" caused the error I mentioned above to be thrown.

On the 5th clean install, I checked the language drop-down box in Admin > Site Settings > Advanced and it was blank. Makes sense then that the DB wouldn't accept a "NULL" value since there wasn't a value there. Not sure if the value wasn't written to the DB during install or what, but it does, at least for me (5 times) not write a default language value to the DB. I clicked "cancel" after seeing that. Next, I went in to the Admin > Languages tab and set the language there before doing anything else and no problems after that.