"Unol Daleithiau America"
Translation:the United States of America
There is no need for yr to be added to Unol Daleithiau America in order for it to be translated into English as 'the United States of America'. Since 'America' is a proper noun, Welsh does not need to add a definite article in front of it.
The English and Welsh genitive (possessive) patterns are quite different in structure.
Here's another way to think about it. The English phrase The United States of America is a long form of America's United States, in much the same way that the hat of Kathleen is a long form of Kathleen's hat. The English short form omits the definite article, and Welsh does likewise.