Taking implementation from sc/source/filter/excel/xestyle.cxx
GetNumberFormatCode(), slightly modified to ensure valid conversion and
force en-US locale data. Also don't unnecessarily convert if format is
for system locale and system locale is en-US.

Change-Id: I9223eaa655132b4106a35c94cb0005559d7575b1
(cherry picked from commit 2011b541)