An Apple device's language (Settings > General > Language & Region > iPhone Language) and its language setting for Siri (Settings > Siri > Language) can be different from each other. This is useful for bilingual or multilingual households, people learning a new language, expats living abroad, etc. For example, you can have your entire iPhone interface (menus, apps, and text) displayed in English, but configure Siri to listen and speak only in Spanish, Japanese, etc.
But if you want to get Siri's new AI features to work, like writing tools, notification summaries, or contextual Siri, both the iPhone system language and the Siri language must be set to an Apple-supported Apple Intelligence language, though they don’t need to be set to the same supported language. If you set your iPhone’s system to a language that Apple Intelligence doesn't support yet, even if you set Siri’s language to one supported by Apple Intelligence, the Siri AI LLM features will automatically turn themselves off, and Siri will revert to its older legacy mode, since Siri AI’s underlying semantic index needs a unified language framework across the OS to read the screen, understand personal context, and perform app actions. As long as both languages are set to supported variations (like US English and UK English, or any of the localized European/Asian languages Apple has rolled out), you can mix and match them without losing any of Siri's new, advanced AI functionality.
Also keep in mind that if your device doesn’t have a minimum of 8GB of RAM (models that do are the 15 Pro and Pro Max, and all later models), it can’t run Apple Intelligence at all, and so it’ll run the legacy non-AI version of Siri. And only devices with a minimum of 12GB of RAM, which currently includes just the 17 Pro, Pro Max, and Air, can use the “most advanced” on-device Apple Intelligence features like more precise local dictation and "expressive voices" (where you can manually adjust Siri's conversational pitch and speed).
