Generally speaking Europe, North America, Central and South America, Australia, New Zealand and any territories/islands these may still administer.
There are many the UK, USA and many European countries are, although some follow different sectors of the Church.
England
Vatican city has 100 percent Christian population, Pitcairn City also has 100 percent christian, Samoa has 99 percent Christian, Romania has 99 percent. Other Countries includes American Samoa 98.3 percent and Malta 98.1 percent.
The population is mostly Islamic, but the government is mostly Christian.
The continents of North America, South America, Europe, and Australia are predominately Christian. Africa also boasts a large Christian population, and the majority of African countries are predominately Christian, however, the Islamic nations of Africa have a larger population than these countries combined. Asia also has a somewhat large Christian population.
No. Christmas is a Christian holiday. And not every country is mostly Christian.
In European countries and countries with a history of colonization by European powers.
Belgium
Countries that have a Christian population have Christmas celebrations. Even countries that do not have a large Christian presence are falling into line with the marketing aspects of the the Christmas season.
Pretty much most of the population is catholic or christian.
Italy and The Philippines
Probably any countries with a Christian or Catholic population
They are mostly christian, the largest denomination is lutheranism
South Africa is a country so I would say South Africa has the highest Christian population in South Africa.