Some languages, like Python, Lua, and Tcl are designed with the intention that you can easily embed the interpreter into some other native program. For example, many VFX applications like Nuke, Maya, and Houdini have an embedded Python script editor that executes the Python scripts in-process so that they can manipulate the host application. Lua is popular as a language embedded in some games.
What was the first such scripting language that was specifically intended to be hosted inside of some other large host process, rather than just being invoked externally by shelling out the way you might execute a typical Bash or Perl script?
Later edit: LISP was mentioned in the comments, but not specifically as an answer. I have accepted the Forth Language as the answer because it is a really interesting language that wound up in all sorts of interesting places, was quite early, and met all the requirements of what I was looking for. That said, it seems LISP definitely at least deserves the "honorable mention" that it got in the comments, and pre-dates Forth quite a bit.
Later, later edit: Be careful answering the question! It is about a language embedded in another program, not a language for embedded systems.