"Ga i gerdded? Os gwelwch chi'n dda."
Translation:May I walk? Please.
The question is "Ga i gerdded?", (please) note the position of the question mark.
"Os gwelwch chi'n dda" is the response, therefore "please" can not be considered the correct translation, as it ("please") is used to ask for something, to give "please" as a response, is misleading as it sounds like person B is making the request.
In English of course, it's the norm to say "If you like", "If you don't mind", "If you see fit", and even the rather idiomatic "If you please", depending on the context, rather that "please"
That is why a more literal translation is appropriate, my suggestion (for this sentence) is "If you like".