AFAICT, the layout is chosen based on the language set in KOReader.
I'm not overly familiar with the layout backend, but that shouldn't be impossible, if possibly rendering the button actually impractical if it ends up switching between 1457 layouts when a random users only needs 2 or 3

.
As for diacritics, while I like the idea in theory (it's basically what any touch aware virtual-keyboard does when shown on an LCD), I'm afraid that in practice, because of the latency of the eInk tech, it'd end up clunky and messy.