Why Did O. Henry Create a German Artist to Paint the Fake Leaf?

Old Behrman is a good example of how a skillful fiction writer will create characters to suit the needs of his plot. This tends to make both the plot and the character seem more realistic. O. Henry's plot required the introduction of a painter who would save Johnsy's life by painting a fake leaf on the nearby brick wall. But the author did not want to raise any suspicion that the artist he created would think of doing such a thing. O. Henry wanted the existence of a fake leaf to come as a complete surprise at the end of the story. The leaf deceives Johnsy and it also deceives the reader.
By making his artist a German who spoke very poor English with a heavy accent, O. Henry could introduce Behrman and show the sentimental old man's sympathy for the sick girl, but at the same time dissuade any reader from suspecting that this old man could or would paint a leaf high up on a wall in the middle of a stormy night. O. Henry intentionally has Behrman contradict himself repeatedly while speaking to Susie. For example:

"No, I will not bose as a model for your fool hermit-dunderhead."
“Who said I will not bose? Go on. I come mit you. For half an hour I haf peen trying to say dot I am ready to bose."

Behrman also expresses contempt for the notion that a leaf could have anything to do with Johnsy's life or death.

"Vy do you allow dot silly pusiness to come in der prain of her?"

Because of the language barrier, Behrman cannot understand Sue completely and she cannot understand him. The reader cannot understand him either--which is exactly what O. Henry wants.
There is also the fact, which O. Henry emphasizes, that Behrman is a heavy gin-drinker. This naturally interferes with his mental processes as well as with his ability to understand and communicate in English. He can't express himself, and he really doesn't know what he would like to express.
Old Behrman is a heavy drinker because he is depressed. He is depressed because he feels he is a failure as an artist. The intoxication created by all that gin will motivate him to make the fatal decision to get a ladder and a lantern and paint that last leaf as a farewell gift to Johnsy. He doesn't care about risking his life. He is a failure anyway. He has nothing left to live for. He puts all his emotions into the creation of a single ivy leaf which has the power to inspire the sick girl with the old man's love and courage. O. Henry describes the fake leaf seen by Johnsy in such a way that it can be regarded as an unusual artistic masterpiece.

Still dark green near its stem, but with its serrated edges tinted with the yellow of dissolution and decay, it hung bravely from a branch some twenty feet above the ground.

It is not just a daub of green paint but a perfect representation of a real leaf which is gradually dying but still managing to cling to its branch. It seems to hang "bravely," and it is this bravery that inspires Johnsy to return to the struggle for existence which we all know and share.