From the submitted press release about a preliminary study:

"Previous research from other groups has indicated that Alzheimer's disease has a later onset in bilingual or multilingual groups."

That's reassuring to any of the many Hacker News readers who know some language besides English. A correlational finding like that, of course, doesn't demonstrate the direction of causation even if it is replicated many times. The kind of preliminary treatment-control finding reported in the press release submitted here would have to be replicated across several matches of experimental conditions and several variations of experimental conditions to figure out which learning programs, if any, reliably result in clinically meaningful increases in sizes of brain structures. (Then the next step would be following up on what the size changes really do for the patients over the long term.) All the usual considerations for evaluating preliminary research reports apply.

http://norvig.com/experiment-design.html

Having noted that, I'm curious about what other languages Hacker News participants speak. English is far from the only language known among us. Languages I have studied include (Modern Standard) Chinese to a proficiency level sufficient for employment as a translator and consecutive interpreter, German for reading professional materials, Japanese, Russian, other Sinitic languages, the original languages of the Bible, samplings of Latin and of various Romance languages, interesting constructed languages (conlangs), and others.

Here's a link to language-learning advice:

http://learninfreedom.org/languagebooks.html

Here's one of my favorite links about one constructed language:

http://www.xibalba.demon.co.uk/jbr/ranto/