Oh that niggles me as well! (even thought I'm a big fan of Scandi drama as well).
But back to Welsh material DVDs....um, couldn't S4C give, or at least sell, to the DVD production companies, (for not much more than what it cost to produce), the subtitles? At the end of the day, it's just a bunch of text, and I can't believe it would cost hours of labour to edit in.
I've seen subtitle files (at least in some formats) and they have time data built in, so there isn't any actual editing really to be done. (OK, I don't know exactly what's involved in creating a commercial DVD, but I simply don't believe that, technically, this should be such a big deal.
Given that this is (let's be honest) such a niche market) can't a bit of commonsense and goodwill take precedence over commercial interests?