"The six nations championship."
Translation:Pencampwriaeth y chwe gwlad.
Because that would be two 'the's in the sentence.
The literal translation is 'Championship of the six nations'
Possession in Welsh is indicated by the position in the sentence, unlike English which often adds 's to nouns.
The other way you could write it might be
Y Bencampwriaeth chwegwladol = Which would work if 'chwegwladol' was an actual adjective rather than one I've just made up. (If it existed it would mean 'sixnationy')
So you can see the above solution is the only accurate translation of 'The Six Nations Championship'