Text setting

Currently it is not possible, @Voice app reads the entire text in a file or article with one voice (e.g. English voice or Polish voice). To some extent it would be possible on Android 8 (Oreo) devices when using Google TTS voices. Google recently introduced an experimental feature there to automatically recognize language and use separate voices to read text in a multi-language file. But for this to work reliably the phrases have to be at least 4 or 5 words long - it's a statistical process of guessing the language, for single words may make a lot of mistakes.
 
Technically it would be possible with some annotations that tell the reading program in what language a range of text is. But at this time I don't have any support in @Voice app for such annotations. I may add this in the future, if more users request such feature.

For reading fluently whole sentences, where only a word or very small fragment is in another language, switching voice would not make sense and sound very unnatural. It would be like one person saying part of a sentence, with an intonation like the sentence ends there, just before the foreign word. Then another person reads that word. Then the first "person" starts reading the rest of the sentence etc.