If salvation comes through Christ alone, then how will those tribes and races who never got to know Jesus Christ obtain salvation?

This is a great question! The Catechism of the Catholic Church states that “Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience – those too may achieve eternal salvation.” (p. 847) 

So it is possible that those who have never known Christ may still be able to obtain salvation since God has many ways that are not known to us. This being said, since we know Jesus Christ, the Son of God and the path He has revealed to us to know His Father and to attain friendship with Him and thus eternal salvation, it is imperative that we not leave any persons or peoples up to chance that they stumble through the difficulties of life and the confusions of doctrines and natural religions and perhaps find salvation. This is why in the very next paragraph, the CCC says: “…the Church still has the obligation and also the sacred right to evangelize all men” (848). No wonder Scripture exclaims: “But how are men to call upon him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without a preacher? 15* And how can men preach unless they are sent? As it is written, “How beautiful are the feet of those who preach good news!” (Romans 10:14, 15) 

If we cannot resist sometimes finding a good store, or having come across a good show that we share eagerly with friends and neighbours, how much more eager must we be to share the good news that leads to eternal life?