Witaj na pokładzie, Szlezwigu Holsztajnie.
There is a special version of OLC GUI prepared for GWX 2.0, you'll find it here:
http://www.subsim.com/radioroom/showthread.php?t=124639
(first post).
I have no idea about "freecam modes", as I don't use freecam.
Polish version of GWX should work well with GWX, but after installing GWX you'll loose possibility of playing in Polish. You'll be still able to select between German and English (and Frech IIRC). I'd suggest to run "English text", "German voice", unless you speak German and you're able to run the game entirely in German.
Which edition of SH3 do you have? If it's "Kolekcja Klasyki", I'd have some tips of getting rid of the initial screen with the loud Piotr Fronczewski's voice.