Common Tongue, English, Common, Vernacular, and Normal Talk are all the same thing. It is a central language which binds all of the races of the world together, and if you wish to be a part of modern day society, you MUST learn common. Nearly everyone you meet will know this language, and if they don't, then they have not been in close proximity to a schoolhouse.
Common in KritanaEdit
Common Tongue is by far, the most useful tool you have at your disposal in Kritana. As a huge center of trade and commerce, it is foolish to not learn it.
Most books are written in Common, and most signs. Most declarations and letters and messages are also given in Common. You should DEFINITELY learn this language.