Official way - create a new project and specify the language that you need.
even with @AlexKaras solution you may have to do much work to get things in up and running.
each language has it's own grammar. If you are going from A to B to use features of B, have to re-write code using B.
> even with [...]
BTW, this is one of the reasons why personally I prefer to use helper objects provided by TestComplete instead of those provided by this or that scripting language when possible. (E.g. aqString instead of string.XXX in JScript and string functions of VBScript.) If you don't work for the same project for years, the use of helpers provided by TestComplete makes it much more easy to port your test code between projects that use different languages. (Yes, I am aware about script extensions, but due to their nature it is not always possible to move test code into them.)
But I am sure someone can tell me a way to check what script language is being used by the project ?
Nice question. As far as I know, after the project was created, there is no place, where the scripting language setting is stored. And maybe it's a good matter for the feature request.
However you can open any script file, analyze the code and solve what the language it is :-)
> what script language is being used by the project ?