I've complained verbally at Tesco in Caerfyrddin, but I'll have to make it official.
The Self Service machines can be switched to Cymraeg (at least they could the last time I used them), but they now have this 'Scan yourself' as you go round the shop system. It makes shopping quicker as you just scan the item and put it straight into your shopping bag. When you get to the machine (special ones for this functionality), they upload everything from your scanner, you pay, and off you go.
BUT the new machines don't have a Cymraeg option. They just do everything in English without any choice at all. I'm so used to doing my shopping in Cymraeg that I got quite confused when the machine started asking me things in English!