Best practise for non-us formated numbers in parameters

I have a contrller that receives data (by POST requests) from an
external system. The external system formats the number according to
danish traditon, that is PI=3,1415, and 3/4 = 0,75.

This gives me problems in my activerecord when specifying these
attributes to numeric, that is using
validates_numericality_of :danish_number
in my AR model. Even when that is solved, I also want the AR framework
to automatically coerce these values as numbers (not strings), so when
the INSERT statement is generated, the DB will not generate errors.

All my other contrllers and AR models use standard formatting.

What is the best approach? I would like to specify this on the AR
model level.