Ahh, now I know the proper term. Thanks for the clarification. 
@Carlos.R I actually just referring games in general, but I think the “common language” in various stories is the one we speak in real world, i.e. English. But then, it depends on the scope of the publishing and the target audience, be it a local region, world-wide, or even for personal use.
I just wondering, should I do some research if I wanted to put the “non-common language,” or can I just slap some words, play around with random gibberish? Although I’ve to admit, using random words can be quite dangerous.