Damn, seems like there is no way to change it the Mac version of Excel
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Changing Formats in the International System Pref pane has no effect on Excel.
If all you need to do is diplay the numbers with the comma separator, I've put together a simple
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formula that will do the trick. The way I have set it up here is to display 2 digist after the comma, but you can easily customize it to suit your needs.
=
FLOOR(A1,1)&","&MID(TEXT(ROUND(A1-FLOOR(A1,1),2),"0.00"),FIND(".",TEXT(ROUND(A1-FLOOR(A1,1),2),"0.00"))+1,2)
A1 above could be any cell reference