String interpolation on the result of a function that returns string

I have a function which gets some text from an xml file like this:

var text = Translate("/foo/bar");

Now in the XML I want to put in string interpolation constants like this:

<foo>
  <bar>Hi, {firstName} {lastName}!</bar>
</foo>

So, I want to do string interpolation on the result of the Translate-function, that would be something like:

var firstName = "John";
var lastName = "Smith";
var formattedText = $Translate("/foo/bar");

But that doesn't seem to work. Is there any way to to this, except reverting to string.Format ?

It could be nice if the dollar sign could be placed in front of anything that is a string, be it a variable, a string or a function, to do string interpolation.

It's worth mentioning that I don't have control over the code for the Translate-function, since it's in a third party component.