I've just detected a misspelled "natürlich" — written like "natürlisch". Is it a mistake or slang/dialect?
If somebody writes “natürlisch” in a phrase like “FIFA natürlisch!” it's virtually certain that it was done intentionally.
It's such a common word that it is not likely to be spelled wrong by genuine mistake.
On the other hand the context let's you expect such kind of jokes. It's common to spell “natürlich” formally incorrect as “natürlisch” to hint at certain sociolects or even just to emulate the slurred speech of drunk people.
Adapted from a comment:
It can be both. In standard German, it's natürlich, but in some dialects and sociolects it is pronounced natürlisch. It may also be spelled like that if one wants to reproduce this pronunciation in writing.
natürlich
, but in some dialects and sociolects it's spoken likenatürlisch
, which may also be written like that if one wants to emphasize this way of speaking it. – Toscho Jul 12 '17 at 19:15