These are common problems for travelling teachers, even in new apts. The causes and solutions are really simple.

The bad smell is usually sewer gas. In newly developed countries sinks often drain DIRECTLY to the sewer. There is no water-filled S-shaped air trap in the drain. None was ever installed. The drain pipe is just a straight tube. I have seen these faulty drains in most apts. in Korea. The builders don't know any better. What can you do about it?
1.You need to buy and tape or screw on an S-section metal or rubber tube. Can you picture it? If you cannot do this yourself you have to hire someone and get help in the foreign language.

2. A cheap alternative I have used is to put a plastic half-filled water bag over the drain. This blocks the sewer gas from rising up into your apt. Take your shower or face wash and then lift the bag when you need to drain the floor. It works fine.

My kitchen sink pipe was not physically connected to the floor drain pipe so I just joined them together with electrical tape. The glue on the tape is not water soluble. This works fine. You still need to leave the rubber plug in the sink at all times with a little water or else sewer gas will come up the sink hole. Sewer gas can make you sick. Take this advice seriously.

ROACHES: Taking the above measures will also keep roaches from invading your apt. from the sewers. Place waterfilled plastic bags over floor drains and keep the rubber plugs in the sink holes.

Roaches will also come under your front door or through open windows if they smell food or decay inside. It is best to put food wastes in a large widemouth jar with a screw-on lid and keep this in the fridge before you empty the garbage. You could use a plastic bag too but they often leak in the fridge. NEVER put food waste directly into an open garbage can. It is a smorgasborg for roaches at night. Disposable food containers need to be washed out and dirty paper of plastic food wrappings need to be put in sealed bags. The key idea is to deny any food to the roaches.

Get used to killing roaches as soon as you see them. I use my hand because I am not so squeemish. If you let them get away they will surely breed. The war is never-ending but you can reduce the problem greatly with these eco-friendly precautions. It is best to avoid using pesticides in your home. You live and sleep there so you will certainly absorb most of the poison.