Let's say a young Russian student is composing an essay and decides to write “I want to earn a billion dollars” in Russian. He knows the word for million is миллион, so he figures a billion must be биллион, but, being an enterprising student, he quickly double-checks his Russian dictionary. He is pleased to note that the word is exactly as he expected, so he writes «Я хочу заработать биллион долларов.» Alas, he has made an error. Even though you can find the word биллион in Russian dictionaries, people rarely use it. Instead they say миллиард:

If you are translating from English to Russian, you must be quite careful if the source document has the word billion in it. In the US the word billion always means 1,000,000,000. That's not necessarily true in other English-speaking countries. For most of the nineteenth and twentieth centuries in England the word meant 1,000,000,000,000. (In the States we call that a trillion). The US system is known as the “short scale” system of naming numbers, and the old British version is known as the “long scale.” In 1974 the UK officially switched from long scale to short scale, but there are still people in Britain who use the word the other way. That means that a good translator will take the time to determine the nationality of the author and the country in which the source was published before he finalizes his translation, and even then it's a good idea to see if the source document has some internal confirmation of which meaning is intended.