I live in a country where the elders of the language decided that we don't format floating point numbers with a decimal point but use a decimal komma instead.
So PI is not 3.14 but 3,14 instead.
Just imagine what pain you have to go through if you want to parse a .csv file written by an application that "tried to do everything right and use locale".
Microsoft Excel will helpfully use semicolons as field separators in this case, making parsing CSVs that came out of it in an unknown locale situation even more fun!
Well, there are some exceptions, like actually caring about the original datetime, but they are rare.
How would you write 3,141.592?
if it's money, you use the . as the decimal separator everywhere. if it's just a number, in the French parts, it's , in the German parts the .
The thousands separator is ' everywhere