Well, I don't think that there is any country where 100% of the people speak one language. There are different people and different cultures in all countries, and people travel and re-locate, and you can usually find people who speak many languages no matter where you are. However, in terms of an "official" language... some countries only have one for a couple of reasons. One reason is purely financial. It costs a LOT of money to publish everything in two languages. Another reason is the demographics of the country. If almost all of your citizens speak the same language, then that tends to be the official language. Some countries are less diverse, language-wise than others. Not that there aren't other language, but just that fewer people respresent different languages.