Also, to add a historical note.

The question words in Slavic. There really did used to be a vowel there after the к and г. It was called a jer, and was either an uh-sound like schwa and spelled ъ or an ih-sound, spelled ь. We know this from ancient manuscripts, where we find written къто.

The question words were either kъ+ word or kь+ word. Examples
къто who
къгда when
къде where
кънда where to (куда); I'm not sure how to represent this nasal vowel see analog in Polish.
кьто what (early in Slavic before the breakup ki became ch, so all Slavic languages have Ч there)

как how is the only one that doesn't seem to quite fit. Any Slavic language historians reading this?

Oh, here you go. Just googled къто and came up with the tale of Boris and Gleb, a good old favorite I read in grad school. Complete with all the jers everywhere. http://avorhist.narod.ru/source/boris.html

This isn't really real, because, as unreadable as it is, it is written in modern Russian style - no "i", no jat' no jus, etc. Do your own search on къто and read the real Old Church Slavonic literature - enjoy!