The Chelish dialect of Taldane has been optimized by something akin to natural selection, over the last decades, to no longer contain openly uncomplimentary meanings of 'Hellish' among the bourgeoisie whose children sometimes become Security wizards. In the first couple of generations, things became hellishly challenging, hellishly relentless, hellishly sadistic and cruel. Not hellishly bad. You didn't say that where an Asmodean priest could hear you, meaning, anytime, and your children grew up not hearing it used that way. In the couple of generations after, Infernal loanwords into Chelish-Taldane started to displace less precise, less useful, overly general terms like the Taldane 'hellish'.
Three of the five other languages Keltham can anchor his mind and Tongues spell on, now, have adjectives that obviously sound a lot like 'Hell' and mean 'torturous' 'painful' 'hope-crushing' 'soul-destroying' 'very extremely bad' -