According to the D20 system the game was based on, one has to have a modicum of Force sensitivity to see a Force ghost. It takes slightly more than that to understand what it's saying. HK-47 and Carth are the only non-Jedi party members who make any note of Pall's presence. HK-47 only comments that Pall shows up on his sensors as a strange reading. Carth can not only hear Ajunta Pall, but understand what the ghost is saying.