It's disappointing (for me too), but I don't think it's possible to run it in Polish language right now under ScummVM.
As I see, Polish dialogs are in main executable, while ScummVM uses *.dat files as replacement.
I took a look into teenagent.dat and unfortunately, there are only English dialogs.
So, unless someone will rewrite dialogs in Polish from teenagnt.exe to teenagent.dat, it's a no-go.
It might be easier to keep dialogs in English and voice in Polish, but that's not done yet, too.
I may take a look at it in next few months, when I'll get more familiar with ScummVM code and I'll get some free time.
For now, from ScummVM work, I've just made support for Polish language for Legend of Kyrandia 1 and I'm still working on Polish version support of Legend of Kyrandia 2: Hand of Fate.
friend have You find time to fix New Teenagent to play on scummvm with voice and subtitles PL?