First off, I use my iPhone bilingually; Dutch and English. Dutch being the main language the iPhone is set to and English accessed via the language rotor.
I had a bit of a shock after installing IOS 6. What happened was that the Dutch voice seemed to take on English pronunciation rules, giving me a rather muddled and odd experience. After doing a reboot the pronunciation seemed to be back the way I was used to, until I used an app that didn't support Dutch, so automatically defaulted to English. When switching back to the Dutch voice the pronunciation issues were back.
Not very impressed I set my iPhone to English as the main language and Dutch accessed through the rotor. This gives me the compact Dutch voice, which I'm not very happy about, however, no pronunciation issues have occurred since then.
Apple accessibility is aware of the issue and is working on it. However they can't give an estimate when it'll be fixed, due to its specific nature..
I am aware that most of you speak English as your first language, so wouldn't run into this issue. However, I felt it was necessary to post this as it would've been a serious deal breaker were I not fluent in English. Did anyone else who has their iDevice set to a non-English language experience this issue?