in the data assist insert record behavior, when you bind the column to a form element, the code in the value field will look like:
<?php echo((isset($_POST["textfield"]))?$_POST["textfield"]:"") ?>
you will need to use the php str_replace function to convert the comma to a decimal:
<?php echo((isset($_POST["textfield"]))?str_replace(",", ".", $_POST["textfield"]):"") ?>