No, not when one has been declared. But in the case of the US, there has been no declaration. If I was traveling from Africa to the US and asked "What language do they speak there?" the answer would be "English." Not, "Well, there is no official language so (blah blah blah)..." It's funny how people get so upset when Mexicans speak Spanish here. They say "This is America! Speak English!" If it looks like a duck, walks like a duck and quacks like a duck, then it must be a duck. There is a law that if you stay on someone's property (say plant a garden somewhere in their backyard or whatever) for like 7 years and they never tell you to leave, then it is officially your property. No documents or agreements necessary. It was implied. Centuries of speaking English, writing laws in English, etc. has made English the official language of the United States, written or not.