Language of the Dust quest for the Serilian language starts in Steamfont Mountains in Gnomeland Security Headquarters by talking to Lingualogian Roosey, a female gnome in a building just south of the Akanon Inn, at -636, 175, 1094 .
You need to examine 15 Deepearth Runes (uncommon drops from any Kobolds, Bugbears found anywhere in the world) to complete the quest. Originally, you didn't have to have the quest for these items to drop, however it now appears that you must.
The Heroic Kobolds in the craters in Steamfont Mountains or the Kobold camps in Butcherblock Mountains seem to have the best drop rates. For higher levels the level 80+ mobs found in various zones (including raid zones) seem to have a much higher drop rate.