Slanguage is language which involves the use of non-standard words, i.e. words restricted in use either to non-formal occasions, or within a particular social class, age range, or outlook.
Yep, so it's exactly the same thing as slang, then.
Word Not To Use Today: slanguage. This...but you can tell how this word is put together. The word slang appeared in English in the 1700s, and it looks as though its meaning started off as a strip of land, often a meandering one, and then later the similar sort of route taken by a tramp or band of strolling players. Later still the special language used among these itinerant people also came to be known as slang.