What is the Six Nations championship? Never heard of it. Football? Rugby? Net ball?
It is a major international rugby tournament. The details will be there if you look on the web.
Shouldn’t “Six Nations Championship” without the article also be accepted? I agree that in many cases you would have to add it in the translation, but this is in isolation. Also, aren’t there also contexts where you wouldn’t, for example if it’s a lemma in a dictionary or if there is another genitive attribute: ”next year’s Six Nations Championship”.
On it's own like this, the pattern translates with the initial 'the' in English.
Remember that the 'actual' translation is 'the Championship of the Six Nations' and that is what needs to be the base form when considering it in other contexts before transforming it into part of a polished English sentence.
You could try looking for specific examples of how it is used in articles on the BBC, S4C or other sports news - several Welsh-languages sites will have the same story in English as well.
Why is "Championship of the Six Nations" not accepted as a valid translation? Is it just because it isn't said that way? As an American with little interest in sports I have never heard of it.
It's the way possession is expressed in Welsh.
This video explains it:- https://www.youtube.com/watch?v=OUuvCwOHs2c
In this context:- 'The six nations' championship' has to be written as 'The championship of the six nations'
The first 'the' and 'of' are omitted:-
'championship of six nations', the remaining words are translated.
The process is reversed going from Welsh to English.
The 'six nations' championship' is the biggest rugby tournament, after the Rugby World cup. Rugby is getting more popular in the USA, particularly womens' college rugby. The American men's team, 'The Eagles' have a close rivalry with the Canadian team.