I speak tiny amounts of another fairly useless language--Irish Gaelic. When I saw Brave this weekend, I was able to use that to understand a tiny amount of Scots Gaelic that appeared in the movie. There are plenty of languages where you can speak one and understand at least a little of another. I also think there's anthropological data that goes missing when we lose languages. Irish has no word for "to have," for example; possession is a concept that is difficult to express in it. The class I took it with got into a whole big discussion about how that had influenced the relationship between the Irish and the English that we would not have been able to have even we hadn't had that knowledge.
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"