"Ga i hufen os gwelwch chi'n dda ?"
Translation:May I have cream, please?
There are in fact a number of variations, all of them correct.
'Os gwelwch yn dda' and 'Os gwelwch chi'n dda' are exactly the same sentence only with the addition of the pronoun in the second version. (lit:- If you will see well)
'Os gwelwch chi fod yn dda' has a slightly different literal meaning:- (If you will see to be well)
There is also the informal Os gweli di'n dda and the increasingly common loan plîs