In the language of the Tuvan people, khoj özeeri means not only slaughter but also kindness, humaneness, a ceremony by which a family can kill, skin, and butcher a sheep, salting its hide and preparing its meat and making sausage with the saved blood and cleansed entrails so neatly that the whole thing can be accomplished in two hours (as the Mongushes did this morning) in one's good clothes without spilling a drop of blood.
That's from this National Geographic article, which analogizes lost linguistic diversity to lost biodiversity. Here's the kind of knowledge we might lose:
Smaller languages often retain remnants of number systems that may predate the adoption of the modern world's base-ten counting system. The Pirahã, an Amazonian tribe, appear to have no words for any specific numbers at all but instead get by with relative words such as "few" and "many." The Pirahã's lack of numerical terms suggests that assigning numbers may be an invention of culture rather than an innate part of human cognition. The interpretation of color is similarly varied from language to language. What we think of as the natural spectrum of the rainbow is actually divided up differently in different tongues, with many languages having more or fewer color categories than their neighbors.
For more on the ties between language and color, check out the amazing recent RadioLab podcast, "Color." And FlowingData features a cool map showing the geographic clustering of endangered languages.