The Widow Harun
Chondathan is one of several native languages used by the various human ethnicities in Faerun. It is the native tongue of both the Waterdeep and Western Heartlands regions and a few other places. Most notably, Chondathan is also the ancestor of Common, the ubiquitous trade language. Chondathan is written in the Thorass alphabet.
In practical terms, all business in the marketplace and between foreigners is conducted in Common. For all other uses, including city signage, humans in Faerun prefer their own native languages. The various languages are discussed in more detail in various Forgotten Realms products, most usefully in the Forgotten Realms Campaign Setting, the Player’s Guide to Faerun, and the Races of Faerun supplement.