You can download and install "Text to Speech" languages from Nokia:
E.g.: for german I downloaded and installed "TTS_German.sis" and "TTS_Nicole-German.sis"
I left the language in "settings -> phone -> speech" untouched (english), because its my main phone language.
The "Msg. Reader" detects the language for text messages automatically and reads my german sms with Nicole's voice.