This seems like a joke to me, or if it isn’t, an extremely racist and close minded approach
If a person's idea of a joke is to put that sign up in the block of flats where they live, for their non-UK native neighbours to see, I submit that they'd have to be pretty racist and close minded to begin with.
True.
But people have specific sense of humour sometimes. Also there are stupid people. Combine the two and there could be a possibility of this being a joke.
That's not a joke though. It's a oblique threat to anyone that doesn't speak the Queen's English.
Which is a joke in so much as the rife grammatical errors in that racist diatribe mean that the author is unable to complete the task they're setting out themselves.
If a person's idea of a joke is to put that sign up in the block of flats where they live, for their non-UK native neighbours to see, I submit that they'd have to be pretty racist and close minded to begin with.