I didn't see anything in that article that says a ',' or '.' convention isn't recognized by all the official locales supported in WoW.
So unless WoW moves to using artificial, historic, handwriting or polite form conventions
this works quite good for all intents and purposes.
Look at the thousand separator: "," in english, "." in german, " " in french. By the way, the International System of Units recommends using a (thin) space for thousand separator in any case, to avoid any confusion.
local decimal = string.match(string.format("%.1f",1/5),"([^0-9])")
local thousands = decimal=="." and "," or "."
That's not that simple. Though getting a comprehensive list of thousand/decimal separator is quite hard (or I failed at google skill), there is at least three characters involved : dot, comma and space. And usage and norms sometimes differ. Moreover, in some language, digits were grouped by 2 or 4, not 3. There is some background information about this on wikipedia : http://en.wikipedia.org/wiki/Decimal_separator