From the Admin Menu, select Languages

Scroll all the way to the bottom of the page and click 'Add New Language'

Select 'English (United States)' or whatever you'd like from the Language DropDown 

Check 'Enabled'

Click 'Update'

I have this exact same problem. Bug perhaps?

What I did (rightly or wrongly) was go to the SQL Server and edit DefaultLanguage field in the Portals table to Allow Nulls. This doesn't seem to be an issue, as long as you are in the US, I guess, since the default value of en-US is added anyway.

BUT, still any place in the entire system where you select a language, the drop down is completely empty.

Seems like a table or list didn't get loaded in the DNN installation.

Have followed instructions above. Still have empty dropdown on site settings/advanced/other/default language. Site Settings still will not update.

I have stopped and started the web site. Same result.

ModuleId: 358
ModuleDefId: 14
FriendlyName: Site Settings
ModuleControlSource: DesktopModules/Admin/Portals/SiteSettings.ascx
AssemblyVersion: 5.1.2
PortalID: 0
PortalName: [M_M_N]
UserID: 2
UserName: admin
ActiveTabID: 42
ActiveTabName: Site Settings
RawURL: /Admin/SiteSettings.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: https://www.mikemorris.net/Admin/SiteSettings.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 1.1.4322; .NET CLR 3.5.30729; .NET CLR 3.0.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 4422b504-669a-4288-821b-5a1700700023
InnerException: Cannot insert the value NULL into column 'DefaultLanguage', table 'mmn_dnn.dbo.Portals'; column does not allow nulls. UPDATE fails. The statement has been terminated.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Data.SqlClient.SqlConnection.OnError
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: Cannot insert the value NULL into column 'DefaultLanguage', table 'mmn_dnn.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 'mmn_dnn.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.SqlInternalConnection.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 ---
Source:
Server Name: LIAM