To know English language when talk about something
I think it's a continuation of the thought from the first sentence. The narrator was trying to get the firefighters to talk about their "thrilling experiences at fires". I think in the second sentence the narrator is saying that the firemen were reluctant to talk about the experiences, perhaps out of humbleness and so maybe wouldn't say much about those experiences and their work. The narrator is equating the reluctance of the firemen to talk about themselves with not knowing the English language in a figurative sense.
Thank you a lot, you explained this sentence so good!