If I had to choose between hours in ICN or a tour in some language other than English, I'd probably just do the tour and enjoy the sights/experience, if nothing is available for walkups. In truth, I've never done one of these tours in any language, but does the language really matter? You get out and about, and maybe it's not so awful if you can't understand a word of what the tour guide is saying. Even if I did understand, I might be inclined to pop in some earbuds and listed to music or whatever; a few years ago I did the Koridoor JSA/DMZ tour and there was a group behind us on the bus who jabbered incessantly for practically the whole bus ride, even when the tour guide was jabbering incessantly.

I don't know if they have a waiting list for these tours, but I would guess the flake-out rate is significant enough (people value what they pay for) that you probably can get an English-based tour on the spot.