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