[Natfan]

To clarify, that's fifty one thousand, correct? Some countries use a period (.) as their "digit group separator"[0], and others use a comma (,).

[0]: https://english.stackexchange.com/a/110693

[BillinghamJ]

I generally work on the assumption that if it's being used to separate three digit separation, it's a thousand separator. If only two digits and it's a currency with cents, probably a decimal separator.