Seoltang is the local trader's language of Kelfour's Landing, which was widely used in I.C.E. Age GemStone III. It is not a full language in its own right. Merchants would string together phrases in Seoltang and use hand gestures to bridge gaps of meaning. Many of the archaic names used in the game were actually meaningful when translated out of Seoltang, which itself loosely meant "the selling language." It is the only language the Kral will use to communicate with outsiders, because their own language is too incomprehensible and idiosyncratic to be translated. The Kral learn it from their slaves.
It is not explicitly stated how old the language is, or its etymological roots, but it is most likely a recently formed pidgin language formed around Kelfour's Landing. Most of the words have obvious roots in what we call Common. Some of the words would instead derive from the much older Quells language, which is spoken by the Spider Temple zealots, a stultified form of the high man language of Zor.
Words such as Quellburn and Quellbourne have meaning in Seoltang, but would not have originated as Seoltang. Quellbourne translates as "freezing lands" in Seoltang, for instance, but its epithet is "land of the silver mist" (still referenced in the older parts of the Lysierian Hills.) It would make little sense for a kingdom to name itself in an incomplete pseudo-language instead of its own tongue. There is no glossary, however, for Quells. Most of the Seoltang words are given in the glossary on page 63 of the Quellbourne source book. The few that are not are often easily guessed, and some like bellacorn are probably also Seoltang. In this case made out of "belli" (noisy, angry) and "karn" (dagger, horn, pointed stick), referring presumably to its loud bellowing and long fangs. Most of the local placenames were Seoltang.