The film doesn't follow the book closely so and so.
If you do remember, Cazik (diminutive of Cazimir, that was his name) was the one who left his doomed family in the barn; he was the one who brought the torch, in order to set the murderers, captured by partisans, alive in flame (in the end of the film).
His lame attempt in German was the attempt to get spared from the awful lot. His "smile" was a convulsive grimace of horror.
Given that, Cazimir showed himself as a weak, sinful human being - but lest we judge him! I think that was the main idea expressed by this character. The people of Perekhody were martyrs; and simple mortals like everybody of us.
Listen to your enemy, for God is talking

Hi aliza_tvito. I think I've come up with a possible answer:
While writing the script, Klimov relied heavily on a book by Ales Adamovich called "Out of the Fire" (see https://www.youtube.com/watch?v=8KY53cDZT4Q). It's a compilation of interviews with the survivors of the Nazi massacres in Belarus. I got myself a copy and discovered many scenes from Idi i Smotri are based on these survivors' interviews (the shooting of the cow, the granny in bed and many more). One of the survivors talk about an SS officer entering a house and asking "Anybody speaks German here?". One man answered "I do." and he shot him on the spot.
So I guess all the "Do You Speak German" scene means is the biker is looking for an excuse to shoot somebody down, right then and there. I think the man he asks ""Do you speak German?" in fact speaks German, but he plays dumb (hence the smile) because he suspects the Nazi biker is up to something. Granted, he is terrified. Who wouldn't be?
Oh, reading again my first message I noticed the use of the term "guy" may carry a derisive connotation. Blame my hopelessly imperfect command of the English language, it was not intentional at all.
Cheers

The "guy" was possessed by immense terror facing Germans.
Listen to your enemy, for God is talking