Actually, if you want to internationalize for the thousands separator, spaces are likely pretty safe as they are preferred by numerous standards bodies, as well as AMA.[1] That also neatly sidesteps any comma/period confusion for the decimal in large enough numbers, since whichever you see, if there's space grouping thousands, it's that one.

1: https://en.wikipedia.org/wiki/Decimal_mark#Digit_grouping