Since I exclusively use Open Office, I had very little hope that this would work for me but it does without a hitch! I was able to create a number format that shows up as 12,34,56,789.00 instead of the western 123,456,789.00. The trick is to use a conditional format. On my system I’ve called it Indian Currency although I understand it’s also used in Thailand:

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[>=10000000]##,##,##,##0.00;[>=100000]##,##,##0.00;##,##0.00
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Go to: David McRitchie’s Excel Pages :: Scroll down to format #37

If you’re an Excel user, you might want to look at this add-in which has additional features.