I don't want to sound racist, but it's typically because a great deal of fast food workers are foreigner who aren't very familiar with the new language.
People immigrate into country (often illegally), people are poor and have no educational record, people are desperate for money, people get minimum-wage job at Wendy's. Anyone who lives in 'Murica knows that there's a ton of poor Mexicans working at fast food joints, many of which barely speak English.