First, they should have everyone, citizen or not, learn proper Englsih and grammatically correct usage of the language by the time they're 18. Americans seem to think others who don't English fluently are the only imperfect ones of society, while in reality, many of their own cannot properly read, write, or speak clearly/correctly. Just look at the degradation of text messaging, social media, and redneck-speak. Additionally, if citizenship is your solution, then what about those foreigners who'll come into your store who aren't citizens, and don't speak English well? What are you gonna do then?
Side note: I wonder if you could even pass the citizenship exam that many immigrants must study for. YouTube can provide a lot of samples of US citizens who can never pass that exam if they had taken it, due to their own lack of knowledge or care for their own country's past. So much for patriotism.