This surprises me - I live in the Welsh Premier League - even though I have no affiliation to it (barring FM).
I have played as Cwmbran Town, Llangefni, Llanelli, Llangenor and my latest save, Calsonic Kansei SSC.
I have made (this is not an impressive roll call) - Champions League group stages x1, Europa league group stages x 3. Thats about it for major success.
Its incredibly tough if you pick up a game with the lower Welsh leagues via the additional league packs knocking around - but thats the game in Wales. I love the fact that it is incredibly tough - it really adds to the accomplishment.