In Matthew 24:14 (cf. Mark 13:10), Jesus says:

And this gospel of the kingdom will be proclaimed throughout the whole world as a testimony to all nations, and then the end will come.

Many Christians understand this to be a sign of the end times: that Christ's Second Coming will only occur after every "nation" (or people group) on earth has heard the gospel. There are many unreached peoples around the world, the argument often goes, so Jesus won't return until this missions work is accomplished.

Is this the only way to understand this verse? Specifically, if this verse is a true prophecy of Jesus, what biblical arguments are used to show that it has already been fulfilled?