Changing the DefaultLanguage
will affect the new scripts you create. It won't change language of existing scripts. You have to create a new script to benefit from it:
if( bVBScript )
then atScripter.DefaultLanguage := slBasic
else atScripter.DefaultLanguage := slPascal;
Scripter.Scripts.Clear;
Scripter.CurrentScript := Scripter.Scripts.Add;
// proceed