[esm]

It's an unsatisfying answer, but Alzheimer's (and most disease for that matter) is a combination of genetic risk and what you do with your life. It is hard to know exactly how much genes or the environment contribute. There are subsets of neurodegenerative diseases that are almost entirely genetic (Huntington's, some forms of Parkinson's, etc.) and some that are almost 100% environmental (there's an interesting story about a group of IV drug users in San Francisco who developed Parkinson's after taking something laced with MPTP). Most cases are somewhere in the middle though.

Learning other languages or instruments is probably useful from a brain health standpoint regardless of your genetics (even if you already have early-stage AD you can slow progression), but they are just proxies for activity. I would guess that learning something that interests you is more important than what that thing actually is.