Personally I would say it could be.
This raises a question about how correct is correct.
The basic rule is that numbers are followed by the singular noun and thus 'chwech gwlad' is perfectly correct.
However another rule is that the numbers five and six contract to 'pum' and 'chwe' in front of nouns.
Thus 'Five nations' would be 'pum gwlad' and 'Six nations' would be 'Chwe gwlad'
It is true that the contracted forms are slightly easier to say but in the real world there is very little difference to be heard between 'chwech gwlad' and 'chwe gwlad'.
So thanks for pointing this out and it is now added to the alternative answers.