Human knowledge is based around language. When I think to myself, my thoughts come together and are built in English. It's perhaps the most amazing phenomena that something man created artificially is so powerful that it seeps into our brains and becomes a part of every conscious thought we have.
However, the great thing about languages is, they too can communicate. What an English speaker knows can be taught to a Spanish speaker. What a speaker of Chinese knows can be taught to a speaker of Korean. In ancient times, the dieing out of civilizations such as the Mayans may have resulted in the loss of great amounts of their knowledge or just the knowledge of who and what the Mayans were. However, in today's world, this is no longer the case. The loss of a language doesn't mean the dieing out of a civilization, it means the standardization of language to more effectively serve it's fundamental goal: To communicate. You can't communicate with language if you are the only one speaking it. As people switch to languages that are more effective worldwide, the less competitive ones will disappear. That's natural. It does not necessarily mean the loss of all their knowledge, but in reality making them more efficient in their ability to share that knowledge.