I don't know who does it. There are some professionals. Professionals do it for when e.g. BBC or SBS shows non-english movies/series I guess. I wish there was a site where you could put money towards english subs for things!! I guess judging the quality wouldn't be so easy etc. Just in the last few days I've had these subtitle-related issues:

- No English subtitles available for S04 of Rita (danish).. So I made English subtitles for E01, by machine translating existing German and Portuguese subs and using the 'original' German for reference. Turned out pretty great, but took an hour or 2. No way I'd do the whole season like that though.

- Season 2 of Blue Moon (in french) has been out for years, no English subtitles anywhere, seems like there never will be. There's a season 3 too I think, not sure.

- Trying to find subtitles for S02 of Overspel (dutch) there's everything available (but in 1 obscure subtitle site, not easy to find) except E07 and E08. I found someone in a forum asking about exactly those 2 subtitles in January 2014! No-one answered...

I'd be happy to pay for subtitles for things like that that I really want! The cost would be small, I guess, divided by a few or a few dozen or hundred people around the world. Then they would be available online forever, hopefully. Just need to coordinate the effort. That could be a great website.

Also, I helped a bit with some spanish and english subtitles for spanish-language tv shows on viki.com 6 or 7 years ago, which was a pretty great-seeming community-sourced translation effort, but it disappeared, along with all its subtitles. At least most of the content did, it was all asian stuff last time I looked, I think it must have changed hands.