Would a christian still be a christian if they were born in a muslim country?

Religion is your choice of faith. You can be any in any religion you like. Does your family like it, that’s another issue. And of course I believe the family members should accept that their children can believe in something else other than what they believe in.

If that is the ONLY difference in their lives
then
yes, of course, almost certainly.

And human being can become a Christian at any time.

Nope, very likely not. Believers are a direct product of their environment and their family.

Most people follow the religion of their parents.  I could talk extensively on the 4 to 14 window in children ministries to explain  the indoctrination technique. If you are born in a "Muslim country," the chances are your parents will be Muslim.  If you are born to a christian family in a Muslim country you could end up being a christian. 

Yes, because the geopolitical nation where a person is has nothing whatsoever with why a person is a Christian. As testified by many Christians in predominantly Muslim, Hindu or even Atheistic nations.

Yes. It doesn't matter where you are born.

Yes, religion is not nationality

There are Christians in Muslim countries.

As for me, I am a Christian born in a Christian country...but I was not raised Christian.  I was basically an atheist in high school, then I became a philosophy major in college and was pretty much agnostic.  Then I got into buddhism, taoism, the new age, etc for almost 10 years before finally coming to Jesus in 2013.  I am a Christian because God showed up in my life and revealed to me that the Bible is true and that Jesus died for my sins.  I understand that there are many people who are Christian because their parents are, but that is not the case for all of us. 

If that person was born somewhere else, he would not longer by the same person.  So why would you expect that he would come to the same beliefs and develop the same ideas and philosophies?  He would not be the same person.

No one is born believing in God.  (Just as they are not born believing in science or anything else.) They learn about it through their family, culture, upbringing, etc.  If they were raised without an expose to the Christian faith, odds are they would be a Christian unless they convert later in life.  

But then, they would not believe in Western science if they were not raised around it and exposed to it.  But their lack of belief is not prove that science is false and people only accept it because they are brainwashed into it.  

And the fact that most people who are never exposed to the Christian faith do not become Christians is not proof that the Christian faith is false.

You also assume that if they were not raised around the Christian faith, but in a Muslim culture they would become Muslim.  It is also possible that they could become agnostic, atheist, or any of hundreds of others religious ideas.

29 "Howbeit every nation made gods of their own, and put them in the houses of the high places which the Samaritans had made, every nation in their cities wherein they dwelt. 
30 And the men of Babylon made Succothbenoth, and the men of Cuth made Nergal, and the men of Hamath made Ashima,31 And the Avites made Nibhaz and Tartak, and the Sepharvites burnt their children in fire to Adrammelech and Anammelech, the gods of Sepharvaim.32 So they feared the Lord, and made unto themselves of the lowest of them priests of the high places, which sacrificed for them in the houses of the high places.33 They feared the Lord, and served their own gods, after the manner of the nations whom they carried away from thence.34 Unto this day they do after the former manners: they fear not the Lord, neither do they after their statutes, or after their ordinances, or after the law and commandment which the Lord commanded the children of Jacob, whom he named Israel;35 With whom the Lord had made a covenant, and charged them, saying, Ye shall not fear other gods, nor bow yourselves to them, nor serve them, nor sacrifice to them:36 But the Lord, who brought you up out of the land of Egypt with great power and a stretched out arm, him shall ye fear, and him shall ye worship, and to him shall ye do sacrifice.37 And the statutes, and the ordinances, and the law, and the commandment, which he wrote for you, ye shall observe to do for evermore; and ye shall not fear other gods.38 And the covenant that I have made with you ye shall not forget; neither shall ye fear other gods.39 But the Lord your God ye shall fear; and he shall deliver you out of the hand of all your enemies.40 Howbeit they did not hearken, but they did after their former manner.41 So these nations feared the Lord, and served their graven images, both their children, and their children's children: as did their fathers, so do they unto this day" (II Kings 17:29-41).

Many ex-Muslims are Christian even if born in countries where Islam is prominent.
It's been reported that, Iran is where the fastest growing [Christian] churches are, even amongst persecution. Amen.

They can be, but the odds are low since most of those countries force Islam by gunpoint or by taxation.