One must remember that the wording 'Festiniog' was made in a period in time when the british isles including Ireland was known as England. Canada, India, most of Africa. Australia, New Zealand etc.etc. was ruled from London. And the royal familly knew how to behave. So words were anglecised all over the world.
It is now a fact that many towns and cities in England are multi linguistic. That is English, Asian and even Polish etc.
Bellach dim ond swn y gwynt yn chwibian, lle bu gynt yr engan ar cynion yn tincian.