Who is at fault if you sideswiped a vehicle while turning left and the vehicle was on the inside?

You, almost certainly, but possibly them:

In a left-hand drive (right side of the road) country, you should not be turning left unless you are in the furtherest inner lanes. However, you should also not be going straight from an inner lane. Thus, if the lane you were in was marked as a left-turning lane, they are at fault; anything else, you will still receive your fair share of blame.

In a right-hand drive (left side of the road) country, exactly how you manage to turn left and hit someone on a lane inside of you is beyond me, but you will be at fault. This applies even if they were approaching from a different branch of the road: a straight travelling vehicle has first priority, and after this comes right-turning traffic. You should not have turned until the path was and would have been completely clear as you turned.