When somebody does not understand what you are saying as a result of your use of slang.
"Got a ripper gobby yesty arvo. Fair dinkum"
"I'm struggling to understand you due to the slanguage barrier."
Confusing out-of-towners or visitors when using slang from your local region.
Moving to a new city and making friends can be difficult if there's a slanguage barrier.
When a non-native speaker fails to grip the slang implications of something that would otherwise seem reasonable to say/wear on clothing. You know it when you see it.
"That kid's shirt says Dream Hum on the backā¦"
"slanguage barrier"