I think we can assume there will be some damage.
I will guess that the mechanism in the latch has somehow broken or come apart. If the square won't turn even when you take the spindle out, it can't be the inside handle that has jammed.
If you are ready to start causing damage, grab the grey metal square part, and pull it out. This will bend or break the shell of the latch. If it comes out, you will find the square is moulded into a round part with two projections on it, like an anchor. Once it is out, you need to find the long metal part inside the casing, that slides backwards and forwards, and force it away from the doorframe direction. This has the brass latch on the end of it, so if you can force it inwards by half an inch or so, the door will open. The biggest screwdriver you can find or borrow will do. A nailfile won't.
If you can't do that, put a heavy object such as a plank or a piece of wood against the door, covering the handle hole, and touching the doorframe (this is to spread the load and reduce risk of damage to the door) and strike it, hard, with a heavy object such as a big hammer, a fencepost, or the shoulder or boot of a person. This will eventually break away the keep in the doorframe, and a sliver of wood from the frame. With luck you will find it breaks out in one piece and can be glued back in later and painted. This is a fairly cheap DIY repair. Damaging the door would cost more to put right. If you know any burly men, they might do it for you. The plank or shelf is important to protect the door.
If you can get inside the room, it can be opened much more easily using a couple of tableknives or thin screwdrivers.
internal doors and frames (and their locks) are usually quite flimsy and easy to break. From what I can see, this applies to yours. A proper front door and lock would be much harder.
If there is anyone trapped inside the room, especially a child or invalid, call the Fire brigade and they will do it.
