I think English monolingual people have a harder time learning and distinguishing homophones (words with same pronunciation but different spelling) - such as to/too/two, there/their/they're, its/it's, etc. If you know another language and correspond the aforementioned English words with those in the other language, you can see that they become quite distinct. For example, to/too/two in French is à/aussi/deux.

Need to learn the second language and use it over years switching thinking between the two languages. Learning it in university then not using it does not count.

I'm pretty sure that learning Polish made me dumber. After years of effort I still don't speak it fluently and lost countless days of my focus for having a marginal convenience of knowing Polish superficially.

It's not just about delaying dementia, either. I've heard it improves cognitive flexibility and multitasking abilities. Makes sense, you're constantly switching between mental frameworks.

I suspect the benefit is greater when the second language is from a completely different family, and has a different writing system.

I don't think it's bad for your brain.

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