This sentence doesn't make sense to me at all. I can understand someone saying you will phone me in Wales or when I'm in Wales. I can understand "You will speak to me in Welsh.", but "you will phone me in Welsh" I don't understand.
Yes, it is an odd sentence and would be better as a question, ie 'Will you phone me in Welsh?'
I think that still has the same problem. To me the call is just the phone ringing. After that you can talk to them in Welsh
Yes, this is a rather strange sentence, it's now been replaced with
"Faint o'r gloch ffoniwch chi?" = What time will you phone?
Thanks for the comment and pointing it out.