Revelation 7:9:
After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
The NLT has it as "from every nation and tribe and people and language".
Revelation 7:14 explains who this great multitude was:
These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
They were converted to Christianity during the Great Tribulation, and then reborn as immortal spirit beings at Christ's return.
Revelation 20:6 says of these people (and others that were saved):
Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
Throughout the Millennium, these saints will help transform the Earth into the Kingdom of God, teaching the unsaved survivors of the Tribulation about God's way.
In Matthew 24:14, Jesus describes conditions shortly before his return:
And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
With the recent advent of radio, television, internet, etc., the message of the Gospel has spread to the ends of the world.
Most people aren't convinced or converted by it, but people from all walks of life are becoming aware of the message.
When the Tribulation comes, these people will remember what they heard and many of them will realize its truth and will then accept God's way during this terrible time.
As a result, this "great multitude" of end-time converts will consist of saints from all walks of life and from all nations around the modern world.
This variety serves a purpose within the Kingdom.
The surviving people will be frightened and confused, so it would make sense for their guides and teachers to appear to them as people that have a similar appearance, speech, and cultural background.