I'm honestly even wondering if I'll ever do a Polish playthrough this time around. I did it for the last 2 games out of curiosity, and it was definitely worth it for the TW1, but TW2 not really, and considering TW3's VA's will be of an even higher calibre again, I doubt I'll even have the curiosity for a Polish playthrough.
Not to mention with the much higher level of presentation of TW3 (Lip-syncing, VA's, Cutscene Composition, Character Animations etc) it'll be even more important to not have your eyes frequently darting across to subtitles and keep them fully focused on the scenes, soaking up every little last detail of what is going on, emotions, animations etc.
In terms of the language I will stick to the polish, because I know it just going to sound the best in this setting and it's going to have the best lines. Some people are complaining about theatricality of the polish version, but it never bothered me and I think that in the most cases it sounded cooler or funnier this way. There are some missteps, like Saskia, but I am willing to forgive it in comparison to the english dub. I'm not gonna sugarcoat it, in the first game english version just sucked. Most voice actors just read their lines without any shade of emotion (like they didn't know any context), translation was terrible and even after they fixed it, many dialogues were much shorter and worse then in the polish version. The english version of the second game was quite of improvement, but still it often sounded really bland to me. The biggest issue in the translation of the second game was that the dialogues didn't flow as good as the one in the polish. By that I mean while in the polish dub they fluently come from one line to another, in the english sometimes it sounds like the character just started talking out of nowhere about something completely different (drinking with the Blue Stripes, for example).
In the third game, judging by the samples, they seem to avoid all of the above problems, but I can't be sure without the proper comparison. Well, I'm going to watch Chris Odd's Let's Play anyway, so I will have a chance to do so. Doug Cockle still does not convince me as a Geralt, but that's just maybe because I just got used to Rozenek, so anyone who sounds any different from him just doesn't feel right. However, I may check out the Japanese version as well, as from what I've heard, the Japanese Geralt sounds like a really good mix of Rozenek and Cockle Geralts.
I had incredible difficulty trying any other languages during the Witcher 1. I don't understand other language's intonations, or well... in some cases I do, for example, watching people speak IRL or in a live action TV show... but in games it's a lot harder. So for me, the Witcher 1 was English only. But the second Witcher game had me between Polish and English pretty much interchangeably. I still stand by my love of English use in the second game, so I still think English is my favourite -- mostly because I understand the cultural references made straight up, off the bat.
For the Witcher 3, I watched the 35 minute demo in Polish and I'm pretty close on being interchangable between that and English as well -- but I think I might do an entire playthrrough in both languages simply because they are both so delicious. I especially love the incidental dialogue in Novigrad since it comes off a lot less harshly than the English accents. But I do like the variety accents in English in Novigrad, it does give a sense of the diversity of people. I also understand the classes more immediately with the English. But I love the Polish for different reasons. ALL SO GOOD.
I could also do a Japanese playthough, but to me, as a speaker of Japanese, I had to say Geralt sounds like a Samurai to me and that really detracts from the Polish cultural undertones that make the world so unique. If you watch the Sword of Destiny in Japanese, I feel like it's more of some typical advertisement rather than the total baddassery you can get in the other languages. It's not because of the language, but the intonation of each line, I think. Japanese is beautiful, but maybe not this time 'round. Just doesn't... work. For me.
I always play with Polish voices + Russian subtitles, I understand Polish a little, and I love how they speak and how their voice actors played all characters. Also I think Polish version is tougher of all, if we talk about swearing, I laughed so hard when was playing it.
I will play TW3 in Polish, but I started to think about giving a try for English and Japanese versions, because English has some awesome actors and Japanese is just interesting for me to listen.