It's not necessarily a good thing for the container to be able to specify locale. Locale should be picked up from the surrounding system; it's just that unfortunately the surrounding system is usually not configured correctly.

And entrypoints/wrappers are definitely possible from a tarball. Just wrap the executables in bin/, replacing them with shell script (or whatever) wrappers pointing to the real executables. That's what Nix/Guix do for languages like Python which require dependencies to be provided by environment variables (as they don't have a way to "close over" the locations of their dependencies).