Tip: you can add a 'Phonetic' name to a contact, and Siri will try to use it.

So you can do stuff like:

Mieczysława -> Mee-etchiss-wahva Krzysztof -> Kzhyshtof Świętopełk -> Shvee-entoh-pehwk

It's still a bit hit and miss, but works quite well with diminutives:

Mieczysława -> Mietka -> Mee-etka Krzysztof -> Krzysz -> Kzhysh Świętopełk -> Świętuś -> Shvientush