Poking around in the translator at dictionary.reference.com, it doesn't look like Spanish normally uses the word "billion". So it's probably just one of those mixups of language and culture, something like an idiom. The translator makes "thousand", "mil"; "million", "milones"; "billion", "mil milones"; and "trillion", "trillion" (perhaps simply imported from English). Now that I have better clarity on your question, I find that it is still a matter of language, culture, and idiom, but between American English and British English rather than between English and Spanish. In the USA, a billion has nine zeroes. However, the British use the word billion to refer to the number with twelve zeroes, which Americans call a trillion. The British use trillion to refer to eighteen zeroes, what Americans call a quintillion. And the disconnect continues on into the higher order numbers, what Americans call "quadrillion" the Brits call "septillion". Oh dear, we are divided by a common language, aren't we? So you see, if your students in Mexico have learned their English from a British source, then they have indeed learned that a billion has 12 zeroes, not 9. I learned this by looking up "billion", "trillion", and "quadrillion" at dictionary.reference.com: http://dictionary.reference.com/search?q=billion http://dictionary.reference.com/search?q=trillion http://dictionary.reference.com/search?q=quadrillion

Here's the beef, from http://en.wikipedia.org/wiki/Billion "The original meaning, established in the 15th century, was "a million of a million" (1,000,000^2, hence the name billion), or 10^12 = 1 000 000 000 000. This system, known in French as the échelle longue ("long scale"), was formerly used in the United Kingdom and is used in most countries where English is not the primary language. 1012 is referred to as a trillion in the "short scale" system. In the late 17th century a change was made in the way of writing large numbers. Numbers had been separated into groups of six digits, but at this time the modern grouping of three digits came into use. As a result, a minority of Italian and French scientists began using the word "billion" to mean 10^9 (one thousand million, or 1 000 000 000), and correspondingly redefined trillion and higher numbers to mean powers of one thousand rather than one million. This is known in French as the échelle courte ("short scale") and is now officially used by English-speaking countries, as well as Brazil, Puerto Rico, Turkey, Greece and the Netherlands." So following that, one billion should have 12 zeros in Mexico! But not in the UK or the USA. In the UK acts of parliament caused the short scale to be used by government, first for financial purposes, and then for all purposes.