The follwing program gives different output in different languages:

------ fp.c --------
#include <stdio.h>
#include <locale.h>

int main()
{
        setlocale (LC_ALL, "");
        printf("%f\n", 42.0);
}
-----------------

$ gcc -std=c99 ~/tmp/fp.c
$ LANG=C ./a.out 
42.000000
$ LANG=de_DE ./a.out 
42,000000


The C99-draft says at 7.19.6.1/#8:

-------------------
f,F     A  double argument representing a (finite) floating-
               point number is converted to decimal notation in the
               style  [-]ddd.ddd,
-------------------

so the latter output (with a comma instead of a period)  is wrong.

This behavior breaks a lot of programs (e.g. doxygen generates TeX with
measure-units like 1,6666cm)