Would it still be worth visiting Poland if I am restricted to just Warsaw?
Yes, very much so. many prefer it to Krakow and there is plenty to see and do. It's the capital, after all.
Krakow much more and say it is a bit more touristy.
And consequently full of tourist traps and a pain in hot weather.
Also, how easy is it for a tourist to get around who doesn't speak a lick of Polish?
In Warsaw, very easy.
If you come, you'll like it.