What's the connection between Slobovian and the elbow?

It’s just a joke. Slobovian, whether Lower or Upper, is no actual language. Lower Slobbovia (Wikipedia):

is a term used in conversation to denote a place which is underdeveloped, socially backward, remote, impoverished or unenlightened. First coined by Al Capp in 1946, the term has also been used by Americans to refer in an informal way to any foreign country of no particular distinction.

And the notion of processing language in the left elbow is a play on to talk out of your elbow, which means ‘to talk nonsense.’

So the joke about a fictional language being processed in the elbow reinforces the author’s previous point that actual languages are processed in the same particular region of the brain regardless of writing system.