Thank you for the response, but I still think you should accept the answer "follow" because in English, on my opinion, "to follow " is just an easier way to say "to walk behind". So, as in Czech "sledovat" is one of the synonymous for the word "jit" sledovat: následovat , stopovat, jít . Still I can be wrong, and I will accept whatever you'll decide to do. And, also, I wanted to thank you all for this wonderful app, you're doing an amazing job!
FWIW, I would add to VladaFu's and Renardo11's comments that, to my native AmE ear, "to follow" suggests intent in a way that "to walk behind" does not. You are more likely to "follow" someone deliberately, while you may "walk behind" someone simply because the "someone" happens to be in front of you.
I do not mean, by this, to say that one cannot also be "following," as in "walking behind" someone completely coincidentally. But I think the more important point is that the Czech sentence does not use sledovat.
In my humble opinion it is possible to follow someone without walking behind them (e. g. using a car, or in a social network). Maybe it is, in a strict sence, not possible to walk behind someone without following them; Merriam-Webster's first definition of “to follow” is “to go, proceed, or come after.” But “to follow” and “to walk behind” are clearly not synonyms.
I think it is helpful for learners that Duolingo insists on exact ans structurally similar translations. Certainly in the present sentence it is helpful that “behind us” is the literal translation of “za námi.”