The original plan for the GNU extension language had an interesting point of view. Not only it focused at allowing programmers to extend a library using a high-level language instead of a low-level one. It also aimed at giving users the choice of which language to write extensions in.
The idea was to use a Scheme-like language as universal upstream extension language, and then to implement other languages (Python, Tcl, a C-like language) on top of it. Users would then have the freedom of using the Scheme-like language, or any of the other implemented languages.
While the plan led to GNU Guile, it didn't reach the point of allowing users to choose what scripting language to use. However, a possibly interesting use case is SRFI 110 - Sweet-expressions (t-expressions) that defines and implements a Python-like language built on top of Scheme.