Hi, and welcome to the Apple (user-to-user) Community
Re: "Can Siri be multi-lingual? Can Siri learn to listen for multiple languages in the same sentence, ..."
Am no expert in this area, however your questions deserve some response .. perhaps others with more expertise may join the discussion once it is under way.
It seems that multiple languages at once for Siri is not generally supported in iOS 17.
ie: Starting in iOS 17.4 Siri will read messages to you in the currently chosen language, and yet you can change the language.
A third-party (non-Apple) article online (link below) has more details:
https://www.macrumors.com/how-to/siri-read-messages-another-language/
However in India, where multiple languages are spoken in the one country:
You can: Use multiple languages to speak to Siri in India - Apple Support
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Apple introduced using Apple Intelligence with Siri in iOS 18.1 and so there are many changes:
(iPhone SE2 or iPhone SE3 can be updated to iOS 18 ... the newest version is 18.1.1)
Sorry I don't know whether or not Siri can learn to listen for multiple languages in a message.
More info: Use Apple Intelligence with Siri on iPhone - Apple Support
All the best :-)