Translation:I go to the Welsh school twice a week.
It's an interesting variation which we hadn't considered and although the word for weekly is different yn wythnosol the above sentence seems to be the only way of saying twice weekly so that's been added now.
Thanks for the suggestion.
Thanks a lot for clarifying the matter. I suppose that a similar variation might apply to the following two sentences in lesson 5: "Dyn ni'n hoffi yfed gwin ddwywaith y dydd." and "Dw i'n mynd nofio ddwywaith y mis.", where “twice daily” and “twice monthly” could perhaps be added to the acceptable answers?