number的初始化应该在前面完成。下面是代码：

import java.util.Scanner;

public class Main1 {

        public static void main(String[] args) {
            Scanner scanner = new Scanner(System.in);
            int number;
            do {
                number = scanner.nextInt();
                switch(number) {
                    case 1:
                        System.out.println("Language selection");
                        break;
                    case 2:
                        System.out.println("Customer support");
                        break;
                    case 3:
                        System.out.println("Check account balance");
                        break;
                    case 4:
                        System.out.println("Check loan balance");
                        break;
                }
            }
            while(number != 0);
            System.out.println("Exit");
        }
    }

两个变化：将scanner.nextInt()行移动到do内部，并将while条件从number != 0更改为number < 1 || number > 4：

Scanner scanner = new Scanner(System.in);
        int number;
        do {
            number = scanner.nextInt();
            switch(number) {
                case 1:
                    System.out.println("Language selection");
                    break;
                case 2:
                    System.out.println("Customer support");
                    break;
                case 3:
                    System.out.println("Check account balance");
                    break;
                case 4:
                    System.out.println("Check loan balance");
                    break;
            }
        }
        while(number < 1 || number > 4);
        System.out.println("Exit");