That's not quite true.
The C# specification
] gives all number entry in western format: nnn.dddd, and does not allow for other culture numeric formats. So it is legal to write
double d = 6.55;
But never legal to write
double d = 6,55;
Number output when your program is running however is normally in the current culture format: so if your PC is set to Serbian, you should get Serbian format numbers.
You just can't write your code using them!