I'm British - I use "," to separate thousands and "." to separate decimals, but that doesn't make me 'right' - it really is just usage and custom, there isn't anything to really recommend one way over the other.
Actually there is. The reason why a comma is used as a decimal separator in some places is the following. The decimal separator is more important
that the thousands separator, a misplaced thousands separator doesn't matter since it has no effect on the value. Now a speck of ink is more easily mistaken for a dot than a comma. It would be bad to mistake a speck of ink for a decimal separator.
Some people have argued that because the dot is more important for sentence structure it should also be used for the more important decimal separator. However this isn't a very strong argument since there is no reason to use grammar rules for the notation of numbers.
That said, personally I use a dot since I need the comma to separate lists of numbers (e.g. 1.2, 1.3, 1.4 ) and the comma notation for lists is almost
universal.