Should I use the self.SDK.Lang.Get function instead of self.lang("") ?
self.lang is just an alias for Lang.Get() so you can use either. I've noted that in the documentation.
GetRuntime function has double bracket:
That's a typo, I've fixed it. Thanks for the heads up.
I miss self.SDK.IPluginBase, self.SDK.ITypeBase documentations in editor's base classes SDK
These don't provide any interesting methods at the moment so are not documented.
I miss GetScriptInterfaceClass in SDKTypeBase in runtime. Can developers use it?
The official policy is that you should not use any undocumented features. What do you need it for?