You could also use the priority column to do this. For example, you could assign a priority of low to the things you know don't have subtitles. Or whatever. It just has to make sense to you. That way you can look at your PTW list as a whole and see which ones you can't watch because there are no subs and sort by that.
I guess this wouldn't work for anyone who uses the priority as intended, but it might work for you. Or perhaps the priority choices could even be expanded to include something like you suggest. That might make more sense than having an entirely separate list.