Can you do an infinite loop in that separate language? Throw an exception? Write to disk? Start processes? Access the Internet?

If not, you can use this language safely inside the config.

If yes, you could as well use the main language of your application, and not introduce an extra dependency.

There is, of course, always a temptation to take awk, or PowerShell, or Ruby, "because it's already installed on the system", which may not be the case on all platforms.