What language did the Netheril use?

TethorilofLathander

They were powerful humans so they may have used the common tongue, but were taught magic by the elves. Which would be their first language?

Sceptenar

Their first language was Netherese, later they also developed a language called Loross, aka High Netherese, used by the elites living in their flying cities.

Kurumi

Yup.. the language of High Netheril was Loross, while the tongue of Low Netheril and the subsequent "survivor states" was Netherese.

Both of these languages were scribed in the runes of dragons. The living language closest to both of these tongues is Halruaan, although Illuskan, Tethyrian, Chondathan and Damaran all bear traces of Netherese linguistic influence and small pockets of Netherese and Loross speakers survive.

TethorilofLathander

Would the runes of the Netherese be draconic rather than Dethek (Dwarvish) then?

Kurumi

Yes.. draconic.

As a nice sidenote.. many sages in Candlekeep have made the study of Netherese history and archaeology their prime focus and they frequently engage in extensive debate over the minutiae of Netherese life.