The Standardization of Symbols
From language to language, the names of the different units of the SI measure are often slightly different, but the symbols stay the same. So while in English, we know of the metric unit of mass as kilograms, in Spanish, they're called kilogramos. But in both languages, you know a person is referring to a kilogram if the number is followed by the abbreviation kg. Those base units we talked about on the last page all have letters assigned to them, so it's clear the amount being discussed no matter what country you're in.
Even in countries where Roman characters aren't used, such as Japan and China, you will still see the familiar sight of m, km, g, mg, kcal and all the rest. Capitalization is very important, too, otherwise a milliwatt (mW) could easily be confused with a megawatt (MW).