While it's true that on Stack Overflow (and the majority of the Stack Exchange sites) we've standardized on English (using the premise that it's the language of the Internet, a premise I support), that doesn't mean that the countries where English is primarily spoken are the locales of the Internet.
The convention you are proposing is a convention of locale, not language, and has many different representations around the world. For example, there are many places where the comma is used as the decimal mark instead of a decimal point.
That said, I don't know that thousand separators are necessary. There are few places where thousand separators are actually used, and it's probably a better idea to have them not used at all.
In the case you present, I'd prefer it show "5.1k" instead of "5,116".
As another example, the representation of the hour of the day is a locale convention as well, and we've standardized on UTC, which is locale-agnostic (even though it may align with certain locales at certain points).
This leads to the question of what the locale of the Internet is (or, what will most people understand)? We've already identified one part of it (the language, which is English), and I believe that the other parts (time of day representation, and numerical representation) are probably better off being as neutral as possible, in order to have the broadest possible reach (which is the point behind English being the language of the Stack Exchange sites).