If you knew of a language other than C or C++ used for any substantial fraction (say, more than 0.1%) of peripheral device designs, I am confident you would have cited it.
If we're just guessing here then I'd guess there'd be some considerable fraction of Assembler. Especially when you're very constrained. If your total code footprint is 16 kilobytes, suddenly it doesn't seem daunting to think about the actual hardware instructions - there's only a few thousand of them after all.
But people also seem to be merrily using special-purpose high level languages for their domain e.g. P4. What I couldn't find much of was C++. If you had numbers we'd be looking at them, right?
