There is already a fictional language established within the Cthulhu Mythos. It is not a full working language as you see within some canons e.g. Klingon, Common Eldarin.
It consists of snippets gleaned from the gibbering mouths, or poorly scrawled diaries of those unfortunate enough to have even thought of the alien entities of the Mythos. You will see more of this as the storyline progresses.
My last series of MDF logs contains an example of it; as does Victories Ravings, it is somewhat hard to read, is usually written in calligraphy (or blood) and the translations are somewhat open to interpretation in that some words mean several similar things and only the context, or the coolest /scariest sounding combinations of these possible meanings gives you the most logical translation. I think Victory has a vague idea what my log says; off the record.
Not to mention some hardcore fans have described proper pronunciation to usually require something drastic like hysteria, screaming until your vocal cords bleed or cutting up your tongue or other horrific mutilations to yourself. It cant be easy speaking words normally spoken by people with tentacles on their face.
Cultists probably get very offended if you incorrectly pronounce it too When you probably need to disembowel yourself while speaking some words to correctly pronounce them, makes diplomacy somewhat difficult. Although they appreciate the effort when made, especially at dinner time....
So basically ye.. good wholesome family fun there... (if your last name happens to be Addams.)