Translation:I like Carmarthen.
These place names mean nothing to me because I'm Canadian.
...that the speaker is rather fond of the town Carmarthen in south-west Wales.
I meant the place-name, xD.
possibly, "Fort of Merlin":
Wikipedia says that the identification of Myrddin with Merlin is not certain.
Is there a rule when you are "supposed to translate" as opposed to just using the Welsh name?
Use the Welsh name if you're talking in Welsh, and the English name if talking in English :)