As a number of other people have noted in comments, while this is "temporary" there are real safety issues. Essentially you have to follow pretty much all the usual building codes, with the exception of structural ones (since you already have a structure). A few key items:
Emergency exits - With a small room you can use just one exit. With a room that size (10 x 7 meters ~ 33 x 23 feet), you should have at least two exits, on opposite sides of the building. That is primarily for fire safety. There are far too many ways for fires to happen unexpectedly - cooking, smoking, electrical, candles, etc. Even if you rule out some (e.g., a strict no-smoking policy) there are always concerns. That is too big a space to rely on a single exit that could be blocked or simply too close to a fire to be used in an emergency.
Ventilation - That is a pretty big room, so some active ventilation is needed - i.e., some sort of HVAC system. All the more so if you don't have any windows.
Electrical - In a barn you would typically need (US rules and terminology) GFCI protection on all receptacle circuits. The rules may change a bit with a building built inside the barn, so it is no longer a wet and industrial type of area. But you do need to consider local codes and you will need a real panel (even if it is technically a subpanel fed from the main building) which will typically includes:
- Lighting circuit (two may be advisable, with lights alternating s that you can work on one circuit and still have lights)
- At least two general purpose circuits. More depending on how many computers, printers and other equipment.
- Bathroom circuit (unless there is no bathroom)
- Kitchen circuits - this will vary a lot depending on whether you just have a microwave and a coffee maker or a full kitchen
Plumbing - That could get expensive. It is easy to run a big electrical cable between buildings. Plumbing is a bit more complicated, in my limited experience. But if you are going to have a bathroom, kitchen or even just a handwashing sink then you need hot (and a hot water heater), cold and drain. If you already have plumbing in the barn then you need to see if it is sufficient for your temporary office space.
As far as insulation and general structure: There are two different ways to do this. One is to add an inner wall to the barn - i.e., the new building will be the same size as the barn minus 6" all around - and fill the gap with insulation. The other is to build a building in the middle of the barn. The first option gives you the most usable space. The second option might make plumbing and electrical a little easier because you have access to the open outside of the walls.
Depending on how long you need this building-in-a-barn to last, what use you will have for it afterwards and other factors, it may indeed be more practical and cost-effective to rent a portable unit of some sort. Construction companies use them all the time as the on-site office when building a new building, schools use them as "temporary" overflow space, etc. They can be rented ready-to-use - just hook up electricity, cold water and drain. Not generally (in my area) quite as large as you need, but get two and put them next to each other.
