Two things could be happening here. Firstly, I've found that if you put a credit card (as opposed to a cash card) in some ATMs they go all 'touristy' on you and offer overseas languages but not Welsh. I use this as an indicator that I've put the wrong card in - usually the card company will charge you interest on cash taken out with a credit card, which obviously you want to avoid if at all possible.
Secondly, although HSBC are good with their Welsh provision, they often update their ATM software with charity appeals, particularly for the Disaster Appeals Committee. When this happens it's usual for them to forget to include the Welsh option. When the software returns to default, Welsh will come back. I've alerted them to this in the past, but I don't think it registered, and I don't want to get too shouty when it concerns urgent charity appeals.