Executive summary of Sapir-Whorf hypothesis: The languages we speak and think in have certain inherent cultural understandings that affect our perception of things. Reading the whole thing is recommended though.
Agreeing with the author of the submitted article that learning a new language is a wonderful thing, and agreeing with you in your capsule summary of the Sapir-Whorf hypothesis (linguistic relativity hypothesis), I have to respectfully register disagreement with the strong version of that hypothesis. Most people are stuck in their thinking because of evolutionarily developed cognitive illusions that are not language-specific.
Many people can be very creative and think thoughts that have never been thought before by their fellow language-speakers even if they are resolutely monolingual. And many of the features of language that might seem to be the most influential on human thought (e.g., whether or not a language has strong concord for grammatical gender) in practice don't seem to lead to any differences in thinking.
That said, even though the strong version of the Sapir-Whorf hypothesis is wrong, it's still a good idea to learn a new language. That does, at the very least, expose a reader or listener to authors or speakers from a different cultural tradition, and that does tend to result in new ideas and new approaches to identifying and solving problems.
