Interesting! Do you know of any countries that use the underscore? I believe I've seen France(...?) use spaces as a separator, but I've never come across the underscore.
Personally I would like some means of breaking up units of triplets that could be used universally. It's frustrating that there isn't a standard across the globe for such crucial means of expression. It wasn't until fairly recently that I learned that the comma wasn't the universal standard.
I am not aware of any country using the underscore to separate multiple of thousands, I just thought of it because it can be used for that purpose in some programming languages. Ans if it's true that no country is using it, then I guess it could be used as a 'neutral' way of doing it.
Spaces cause ambiguity with separate values like 123 456. Underscores are ambiguous with missing values due to the common use of a line segment to indicate where to fill out form values in addition to the same separate values issue as spaces.
Personally I would like some means of breaking up units of triplets that could be used universally. It's frustrating that there isn't a standard across the globe for such crucial means of expression. It wasn't until fairly recently that I learned that the comma wasn't the universal standard.