I understand that’s probably the best translation option. Making brit-cossackish accents, that’d probably sound like germans in Hogan’s Heroes.
Anyway. I always watch movies in VOST: even if a voiceover is good, could it be better than the original voice? Worse, if a culture is to be depicted, how could that be deprived of the main feature of any culture, the language?
The most reliable way to get the Witcher 3 in VOST (polaki voices, english subtitles and interface – this assumes that you installed the game with the polish data) is simply to edit (somewhere in Documents) The Witcher 3/user.settings with a text editor as follows:
[Localization] RequestedSpeechLanguage=PL SpeechLanguage=PL RequestedTextLanguage=EN TextLanguage=EN