Write well. Write often. Edit wisely.
If a character comes from a country different from yours or different from that of your other characters, make sure that he uses words and phrases peculiar to his country and not to yours.
Speech and thought patterns should differ from character to character. Learn tips for differentiating character voices.
Characters should use words specific to their backgrounds and histories, and not sound like the author who created them. Explore the importance of word choice for characters.