at the beginning of where? the plugin? the entry of the function or the switch case? a const static variable of the function? what do you feel would be the best point?
I also have to admit that I don't like this function very much. According to the wxSplitterwindow reference the particular value should be between 0.0 and 1.0 both included, but it does not even say explicitly how many digits it should have, that is to say what stepping it accepts.
PS: about threads: any manipulation of the locale in another thread would change the result of localeconv() here. But the decimal separator always returns a char, if I understood right, under every possible locale, never an empty.
PS: perhaps a better way would be to parse the double into a wxString manually, composing the string by getting the integer value from the double, add a point, and the decimals from the double, all retrieved separately?