Adding extra language causes "ArgumentOutOfRangeException"

I was testing out my translation mod adding a new language using "language-add.xml". It worked well, but as I exited and restarted the game, the main menu refused to start up.

And I saw this error recorded in the log:

------------------- Error ----------------------------
00:02 - ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
System.Collections.Generic.List`1[Offworld.GameCore.Text.ParsedInfoLanguage].get_Item (Int32 index)
Offworld.GameCore.Text.TextManager.UpdateCurrentLanguageInfo ()
Offworld.GameCore.Text.TextManager.OnInfosLoaded ()
Offworld.GameCore.Text.TextManager..ctor (Offworld.GameCore.Infos infos)
Offworld.GameCore.Globals.Initialize (IInfosXMLLoader xmlLoader, INetwork network, IAppInfo appInfo)
AppMain.Start ()
------------------------------------------------------

Meanwhile, I saw the language setting became bugged in my GameOptionsSave.xml:

What did I wrong, or is that you can't add a brand new language through mod? If so, what is the best way to create a translation mod?