My Little Pony: Friendship is Magic - Why do people think King Sombra can't talk?

He also says "crystals" several times. However, pay attention to the way he says it:
"….cryyyyyyystaaaaaaalsssss….."
Long and droning. He can talk, just not very well. Basically what I'm saying is: a mortal trapped in ice for a thousand years after having his body reduced to fog isn't in the best cognitive condition (either hibernation sickness, brain damage, or something else along those lines)

@TARINunit9

Or he could have gone batshit from the isolation. A thousand years in a pitch-black, underground cavern with nothing to do but fantasize over taking back the CCCRRRYYYSSTTALLLLS will do that.