Interworld Common languages are languages designed, by people from worldgate-affected worlds, to allow communication between the people of those worlds. There are two Interworld Common languages, or versions of the language, on which more below.
Common languages are generally an idea of convenience for authors and their worlds, but in this case they cause problems of their own.
Any sudden mix of isolated cultures requires and soon develops some method of communication - whether a pidgin mix ([definition]) or everyone adopting the language of the most powerful nation. A deliberately constructed new language is unusual, but is explained in this case by a lot of bullheaded politics and a timely pitch from several language geeks eager to avoid military conscription.
The Interworld Common languages aren't perfect, but they are an answer to the question "what language are they all speaking?".
The primary concern with the original version of Interworld Common was to make a language that was both neutral (not favouring any particular language family) and very easy to learn.
Interworld Common languages are shared property among Pro members (as far as a language we never see counts as a thing).
They are also freely taught among interworld visitors. Anyone whose characters or worlds are involved in the worldgate events (or even just play at the inn, etc) can choose to say their people know Interworld Common (or second interworld common, at appropriate time in the timeline) to a desired degree of skill, without too much stressing about the details.
We don't envisage anyone making up a 'real', speakable interworld common language. But hey, if you really want to...
See also: list of languages