This was the title of a well-known radio broadcast by Saunders Lewis. The usual translation is 'The Fate of the Language'. It is also acceptable as the 'The Language's Fate', even if that doesn't really strike the right note in English.
This is due to the fact that 'The X of the X' in English translates as 'X y/yr/'r X' in Welsh i.e. this sort of phrase in Welsh is never preceded by a definite article (so always 'Tynged yr Iaith' never 'Y Tynged yr Iaith').