In the US and UK, numbers have a comma separating the thousands and a period separating the decimal portion.
The median round-trip time is 1,256,532.65 milliseconds.
In much of the EU,
as well as Canada (both French- and English-speaking), numbers have a space separating the thousands and a comma separating the decimal portion.
The median round-trip time is 1 256 532,65 milliseconds.
In much of South America, numbers have a period separating the thousands and a comma separating the decimal portion.
The median round-trip time is 1.256.532,65 milliseconds.
India (specifically Hindi-speaking India) uses a format much like the US and UK, except (if I understand correctly) after the least-significant thousands grouping, additional groupings are by the hundreds. (I must admit, whenever I encounter this format, I tend to assume a digit got omitted unless I know the author's culture.)
The median round-trip time is 12,56,532.65 milliseconds.
In most cultures, the localized number format is used only when communicating in the native tongue (transitioning to the US/UK style when communicating in English). However, this is not universally true.
Of note: Canada is the only country I could find whose official language is English and whose number formatting conventions do NOT align with the US/UK convention. Edit: I've been informed in the comments that the US/UK system is the norm in Canada.
There may be other culture-specific Arabic-numeral number separator formats as well. These are the ones that I found in my research.
These numbering systems can usually be interpreted without knowing the author's culture, but sometimes there are ambiguities (such as the number "one and one one thousandth" vs "one thousand and one", both of which could be written as either "1,001" or "1.001", depending on culture). Sometimes these ambiguities can be deciphered from context within the rest of the answer/question, but sometimes not.
Is there a preferred format for numbers when writing question/answer content on English-language Stack Exchange sites, or is it okay to use whichever format is native to us?
@Rene has dug up a couple relevant links in the comments, I will summarize their relevance here.
This discusses a preference between US and UK spellings of words like "favorite/favourite". The answer says that, for tags, the US spelling should be used. For body text, there is no official preference. For question titles, the most commonly used spelling should be used to increase search engine visibility.
This discusses editing questions to conform to a specific culture's number formatting. The answer says that one should not edit to conform to cultural standards unless there is an official preference, but stops short of saying whether there is such a preference for numbers.
I initially thought my question was a duplicate of this one, but this one discusses how numbers are displayed by the Stack Overflow software (in places like vote counts) rather than what formats are okay to use when writing question/answer content. The consensus on that question is that Stack Overflow should use US/UK number formatting, unless otherwise localized.