I lied for the purposes of the joke. I'm not actually very good at Welsh, although i did do it for 7 years. But, like people have said above, if you never use the language, it leaves you, and i never had any need to speak welsh, so i've forgotten most of it.
I used to live in North Wales, on the Isle of Anglesey, which would make me a Gog, if i were actually welsh....