Olsen v. Nebraska was a court case where a guy named Robert Olsen was arrested because he was teaching German to some kids in a school in Nebraska. At the time, there was a law that said it was illegal to teach any language other than English to kids who were in elementary school.
The law was made because some people thought that if kids learned other languages, they wouldn't learn English very well and that might be bad for them. But Robert Olsen didn't think that was fair, so he took the case to court.
In court, they had to decide if the law was okay or if it went against the Constitution, which is like a big rulebook for how the government should work. The judges decided that the law wasn't allowed because it went against the freedom of speech and the freedom to teach.
So basically, Robert Olsen won the case and it became legal to teach any language in Nebraska schools. Everyone was happy because now kids could learn more languages and have more opportunities in the future.