Decimalcomma instead of decimalpoint

[Gunnar]

How can I automatically get a decimal comma instead of a decimal point in math 
environments?

I've tried
\AtBeginDocument{\def\extrasspanish{}}

and even
\DeclareMathSymbol{,}{\mathpunct}{letters}{"3B}
\DeclareMathSymbol{.}{\mathord}{letters}{"3B}
\DeclareMathSymbol{\decimal}{\mathord}{letters}{"3A}

but no success.
Any better ideas? I wish to have numbers converted automatically.