Because here the "," is standing for what you are using the "." for. For example if you have to pay 3 Euros and 50 Cents that looks like 3,50€ over here. If you want to make a large number more readable you place a "." for every thousend 3000000 is much easier to read when spelled 3.000.000 .
No idea why you have swapped it...(or did we???:hmm
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But you are also driving on the wrong side of the road....
(sorry, couldn't resist:rotfl