This might be an entirely UI bug, or interaction between two UI bugs.
I have Q,W,E,R,T set as hotkeys for selecting characters 1-5 respectively. When on the “Enter Incantation” screen typing one of those letters produces the “switch selected character” sound, but doesn’t seem to update anything on the screen.
Sometimes, when entering an incantation, if the last character-selecting letter typed was *not* that for the character with Linguistics ability, the incantation will fail.
Usually, after beginning an incantation, selecting another character by any means (clicking on avatar on map, inventory portrait, or hotkey) will abort the incantation.
This latter issue makes Linguistics fairly useless in battle.