To know English language when talk about something

There are lines from the book i am reading and want to translate them right:
“Most of the time we sat around trying to draw the men out about their thrilling experiences at fires. But if there is one thing the fireman doesn’t know it is the English language when talking about himself.”

What is the meaning of the second phraze? I can’t figure it out. Thanks in advance for your help.

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.

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Thank you a lot, you explained this sentence so good!