The localisation provided by the language pack for Welsh/Cymraeg is incomplete. I'm sure this is known. I'm filing this partly by way of indicating that it is being used. (There are sometimes doubts about this kind of thing...)
Oh - so best would be if you can register at http://translations.documentfoundation.org , and help translating :-) If you do few translations every now and then when you have a bit of time, it can progress significantly!