When installing macOS Sequoia 15.3, iOS 18.3, or iPadOS 18.3, Apple Intelligence will be turned on automatically on compatible devices, Apple says

Apple Intelligence will only be enabled by default for hardware that supports it. For the iPhone, that's just the iPhone 15 Pro series, iPhone 16 series, and iPhone 16 Pro series. It goes further back on the iPad and Mac—Apple Intelligence works on any model with an M1 processor or newer.

If you travel to a non-English speaking region, Apple Intelligence will still work, so long as English is the language your Mac is set to use. In the European Union, M-series Macs running macOS Sequoia 15.1 or later can run Apple Intelligence while using a currently supported language.