When you go to 'options' > 'units', there are two settings for volume. Batch volume is on the top but partway down the list is a setting for 'mash volume'. Make sure this is set to liters as well.
BTW, as long as you are halfway towards metric, why don't you set the weights to metric as well? It really makes things much easier to deal with. Although, I do have to admit that I favor Fahrenheit over Celsius as being a bit more accurate without having to resort to reading tenths on the thermometer.