Currently, trying to apply a rune with Deep Speech active asks if you'd like to put an Espruar rune on--however, I don't think it gives the ability to spend Espruar runes despite this. It says it can be scribed in many scripts, though it was written early-on in a form of Espruar script when the drow tried to contact aliens.
Either:
let Deep Speech give success for Espruar runes;
change Deep Speech to a runic language for Fiendish runes; or
let Deep Speech give success for both Espruar AND Fiendish runes.
Allowing it to give success for both Espruar and Fiendish runes would allow this secret language to live up to its potential, specifically in that it is written using many different scripts despite the more notable one being the time drow did it.