The Twiddler kind of does this already. It will run your custom config file and, when you press N+S M000, it will load the standard factory default mapping until you cycle power. In development we discussed different schemes to support multiple config files but we decided to go with one custom, one default config for simplicity sake,
Remember that the Twiddler (and any keyboard for that matter) does not switch languages - that is done at the OS level. The Twiddler sends, and is always sending, standard HID codes to the OS. The OS looks them up against the current language table and presents the character on screen.
n-Key Rollover. The current topology does not support that as it does not support how the Twiddler detects its chords and does the lookups. For new versions of the Twiddler, n-Key Rollover could be interesting.