～てくれる always means someone performs an action that benefits the speaker. It can extend to people very close to the speaker as well, but since this was a conversation between this man and a stranger, that element doesn't come into play.
分かってくれなかった means that his English wasn't understood by the stranger at the airport. The stranger at the airport "didn't do him the benefit of understanding" his English... is how you could think about it if you were translating it hyper-literally.
I listened to the whole thing, and the man who replies 上手ですよ is obviously just being nice. Or, perhaps, because he too studied English in Japanese schools, he doesn't understand just how bad Japanese pronunciation of English can be and is being sincere. But saying something reassuring after someone bashes their own ability is kind of a standard thing to do in Japanese culture anyway.
Stumped again....
This exchange is from Genki Lesson 18, listening comprehension C. A Japanese man in London missed his airplane flight home because he couldn't speak English:
「エクスキューズミー。アイハフトテイク、フライト521」って言ったんだけど、わかってくれなかったんです。
上手ですよ。
I'm not sure how to translate "わかってくれなかったんです。" The use of くれる has me flummoxed. Who is it that does not understand? Is the Japanese gentleman not understood, or does he not understand what is said in an English reply? And why does his companion praise his skill in English if he failed?
Thanks for any insight....