I bought a copy of Rock Smith 2014 from an ebay auction which claimed to be an English copy.
I live in England (Uk)
I received the item today and the box art is in polish.
Is this going to be a problem or is it just box art?
It shouldn't be a problem since both countries use the PAL version of games. The in-game language might be an issue, but there might be an option in the game to change that. If you can find it.
There is no PAL / NTSC-US / NTSC-JAP restriction anymore, the only thing you need to check is if your language is available. Either on wikipedia or online gaming database which is unfortunately in german, but easy to understand. on the left you can search by title.