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Yes. Just change the language to Japanese.
Note that Siri is monolingual for now, meaning US Siri cannot understand/speak Japanese, and Japanese Siri cannot understand/speak English. I find Japanese Siri to be more polite. :D
 
Yes you can.
But your Japanese should be, uh, a bit, standardised. I mean, Siri can only recognise standard voice.
 
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