check the rpg_scene file, its in there, but you also need window_base where also is something in it.
in case you want to alter the names on the menu list alone, maybe check
YEP_MainMenuManager
and add stuff you want.
EDIT:
in rpg_scenes, you can find this line:
Scene_Menu.prototype.createCommandWindow = function()
thats where the menu command is written down,
this._commandWindow.setHandler('item', this.commandItem.bind(this));
that line is the command window, sethandler, "item" is the name in the menu
while "this.commandItem.bind(this)" is a function of :
Scene_Menu.prototype.commandItem = function() {
to tell what is happening.
but creating a menu isn't easy unless you know what you are doing.