For me personally, Siri has been pretty good for parsing multilingual pronunciations. I just tried the three ways of pronouncing a rare name in my contacts (this is a name that can be in English, Japanese, or Italian) and Siri got all three. I've noticed some improvements in this area in the past year or so, so perhaps it's gotten better since you last tried it. Another tip would be to say the full name. In the past (when it was much worse at this) I've noticed a better hit rate when the entire name was said.
I admit it has gotten better. The biggest challenge I have seen is when a letter is pronounced differently than in the language being used, such as a soft X sound (Sh); which is not in English. Siri can't seem to find names with an x in it and returns ones with an s; even if I say the whole name. Part of the problem may be that I have contacts that make sense if you are looking for an sh in the name and thus Siri returns the most likely ones. I fixed the problem by simply tagging the name Wife.
Some accents also give it problems, such as zinc for sink; even contextually. I can understand if you use zink stand alone but if you use it contextually Siri gets confused. Speakers who don't aspirate H's can cause problems as well, as Siri thinks hair spray is air spray and returns supply stores instead of beauty stores. Accents and sounds not in the spoken language are no doubt hard to dechiper, and if Siri returned sink for zinc every time more people would be frustrated.
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