Yeah I know you can make batteries of those. That means they have enough free electrons to migrate through the wire, due to some chemical reaction that happens with the lemon fluids along the electrodes and thus create an electrical current (no idea actually; chemistry is the one branch of science that I never understood a thing about), but the acid as you find it in a lemon fruit, still isn't concentrated enough to do damage to your body. If you have some irritated skin or a wound on your fingers and pour freshly squeezed lemon or orange juice over it you will feel your wounded skin burn a bit from the citric acid, but that's all it can do to you. Healthy skin won't react in any way. Don't try that with "real" battery acid though.
The current of a fruit battery is probably only a few mA. You can't light a conventional light bulb with only one lemon or orange, but small LEDs may work or some small electrical device that only uses LR41 batteries. Like a watch or a digital thermometer.
There's some way to put several lemons in line to make a more potent battery. Oh well I had the most horrible physics teacher ever at my school in those years when electricity stuff was on the curriculum. So I have no clue what I'm talking about.