There really isn't much information about the language of the Old Ones. However, Sword of Caledor, the second book in the Tyrion & Teclis trilogy, describes the language of the Slann. They are certainly related, if they aren't the same thing. The White Tower has scholars who can translate it, but it takes elves decades to gain even the basics of the language. You need to understand magic to even have a chance of translating the texts. The characters themselves shift meaning depending on their closeness to other characters. High Loremaster Morelian believes that only the Slann and Skink Priests are capable of understanding the language. It's for a people with a very alien psychology. Caledor achieved the most proficiency of any known elf. Morelian suspected that part of his proficiency with magic came from that knowledge.
As you might image, Teclis did not take decades to learn the basics. He also wanted to master the language after learning that Caledor did.
Edit: Corrected some wording. Also I thought of an analogy. You know how Eltharin is writing with emojis? Slann is that, but even more complex. It doesn't even go left to right, up to down, or vice versa. A smiling emoji can mean tree if it is at an angle of 161° in relation to the closest brick emoji and 5cm away from a backpack emoji. But if it is 4.999999235564cm away from said backpack emoji it will instead mean deep sea predator.