You know, this got me thinking.
The Slaan, the Elves and the Dwarfs were made my the Old Ones, right? And they were taught much of what they know, even if most of it has been lost. Since the Old Ones can be expected to be masters of Qhaysh (since the Slaan, arguably their favoured creations, are such), and since knowledge of magic in WH Fantasy can be expected to ultimately descend from them, would it not be possible that a hypothetical unified language of magic would be the one used by the Old Ones? I mean, I suppose they could have just conjured a bunch of entirely new languages to describe specific newly-made points of view on magic (Elf, Slaan, etc.), but I find it more likely that they simply altered their own language to fit the newborn perspectives.
And that language seems like exactly what we might be interested in, considering our dabbling in Waystones. They were not designed by the Old Ones (if I remember correctly), but they were made by a generation much closer to them than ours. Their construction probably drew heavily on the remnants of Old One knowledge.
Granted, most of this is hypothetical, but it seems plausible.