KLL should support locales directly and output what's expected given the configured locale.
This will be tricky as the planets need to be aligned (KLL, Firmware, USB, USB driver, OS) in order to pull this off correctly. Merging locales together generally isn't possible; however, there are usually "super-locale" available on an OS that supports a wider variety of keys which may help.
This is a long-term project on how to get KLL to output unicode.
As KLL gets more sophisticated it becomes harder to test all possible test cases.
Using a basic Host-Side KLL it should be possible to test KLL using KLL. In otherwords, because KLL defines the input and the output, we can program both the kiibohd firmware and checking software to validate each other.
Needs to support per-cycle processing and arbitrary time jumps. Other advanced features such as interconnect and Pixel Mapping should be relatively straight-forward to emulate.
Get the KLL processing engine working directly on your host computer (cross-platform).
With a bit of work, it would be possible to use this a cross-platform karabiner or auto-hotkeys interface.