Goran Visnjic is Croatian and since Croatian is very similar to Serbian, his lines in Serbian in the opening scene are completely correct and sound like actual Serbian dialogue. The subtitles don't convey the full extent of the dialogue: his lines are full of expletives that don't translate to English very well, but are quite in the spirit of the Serbian language.
Joel and Sheila mention The Tonight Show Starring Jimmy Fallon (2014). Drew Barrymore is close friends with Jimmy Fallon, whom she starred with in Fever Pitch (2005), and co-owns Flower Films with his wife, Nancy Juvonen.