I stay in Bangladesh. I've heard that martin guitars are not suitable for Bangladesh due to the climate. Is it true? If yes, then what should I consider if I'm importing an acoustic guitar since my country has no other official brand shop except Yamaha.
The brand of the guitar doesn't really matter.
Extreme climates and/or climates with high humidity tend to degrade acoustic instruments more quickly. I would recommend putting moisture absorbers (example) in the same room as your guitar is stored. And when you aren't playing it, instead of buckling it up in the case, leave it on a stand, as allowing the air to circulate will reduce the tendency for mold to grow.
To avoid an expensive setup, you should search for a guitar that has already been stored in your climate for a few months. But even if you import an instrument, the physical changes associated with a new climate won't be irreversible; a competent technician should be able to set it up for you once you've given the instrument some time to adjust to the local weather. I talked to a luthier in Hawaii who said that he was always busy making adjustments on instruments that people had brought from the mainland because the humidity caused the wood to warp in unpredictable ways.
I'm just going to give an educated opinion here. It seems to me that if a yamaha guitar is "suitable" in Bangladesh, then other makes and models would be stable and perform well there also. Keep in mind that any instrument needs to be cared for if the climate is hot or dry or damp or cold. Temperature extremes and humidity extremes can cause problems for just about any instrument if not cared for properly. Martin guitars have a reputation of being well made, as do Yamaha guitars, but I'm unaware that Yamahas have a special advantage over Martins in any individual climate here on Earth.
Once you get the guitar, I think as long as the guitar stays in one consistent climate, humidity and temperature, you should be fine. I own a Yamaha guitar and have traveled to many places with it. I just made sure to loosen all the strings to avoid them breaking. This is just my opinion and experience.