Okay, say you allow a driver's license exam to be taken in a language other than English. Where does it stop? According to the 2000 Census, there are 337 languages spoken in the United States. Since the written exam is a government document, if the Supreme Court, be it state court of the federal one, accepts it as precedent then you are going to have to provide every government document in every possible language. What about textbooks and exams in public secondary schools and universities? Tax documents? Job applications (Equal Opportunity Employment Act)? Loan documents (Equal Opportunity Lending)? At some point you HAVE to say no. Where you define the stopping point, or even not allowing a starting point at all, is an arbitrary decision, but for the sake of functionality is HAS to be made. You cannot justify having SOME government documents in SOME languages, but not other documents or other languages. The 2000 Census also showed that 82% of respondents considered English to be their native tongue and 96% considered themselves to have a fairly functional grasp on the language.
Also, please explain how Person A, who cannot read English because said person is illiterate, is different from Person B, who cannot read English because said person does not speak English. When you're talking about a WRITTEN examination they are, fundamentally, EXACTLY the same. At least if you read the questions aloud to the illiterate person they would, more than likely, be able to understand and answer the questions.