As far as I know, you cannot. That value is stored in the MDS file and is not editable. While it's a good suggestion, consider the challenge, then, at converting any script code and code units over to the correct syntax, like from DelphiScript to VBScript.
Out of curiosity, what prompted your determination that you need to change script languages? Technically speaking, the script language used for testing an application really only matters if you're actually going to be creating embedded tests within the application (what's called a "Connected Application"). Other than that, it's a matter of personal preference and familiarity when it comes to determining scripting language.
