We have sailed with Costa to Tunis and the problem isn't that they aren't good with English. The problem is that the English speaking passenger's are in the minority and they often end up canceling the English speaking tours due to lack of participation.
We did a tour that took us to the Roman Baths and the Souks. The Souks were a nightmare I would not choose to go back. Picture an indoor flea market with very aggressive vendors trying to keep you in their little stall or shop and it is like a maze and very easy to get lost and lose your sense of direction, the tour operator is only interested in taking you to his friends carpet store so he can get a kick back on whatever the passenger's buy. One elderly gent was nearly left behind because he didn't want to go into the souks. The driver said he would come back to the spot where he left the man on the way back to the bus, but he never did. D/H and I went and got the fellow and made sure he got on the bus.
Now the Roman Baths... a UNESCO World Heritage site. All I can say is WOW! I could have easily spent the day there. It is amazing how well the Roman's utilized technology so many centuries ago and I highly recommend seeing the ruins there. Fascinating stuff.
If I were to do it all over again, I would see if I could find a local guide that has been highly recommended (english speaking) to take you there. I also understand that even though Tunis is Muslim, they make some very wonderful wines there. I would see about perhaps a tour of a local vineyard, winery as well.
I can tell you unless you are wanting to take home some sort of leather good, berber rug (I did get to see a lady in the process of making a berber rug at the souks) or expensive gold jewelry I would avoid the Souks, the merchants are extremely aggressive and in your face.
Hope that helps.