How to convert a String to another locale?

I suggest you have a ten digit lookup String and replace all the digits one at a time.

public static void main(String... args) {
    System.out.println(arabicToDecimal("۴۲"));
}
//used in Persian apps
private static final String extendedArabic = "\u06f0\u06f1\u06f2\u06f3\u06f4\u06f5\u06f6\u06f7\u06f8\u06f9";

//used in Arabic apps
private static final String arabic = "\u0660\u0661\u0662\u0663\u0664\u0665\u0666\u0667\u0668\u0669";

private static String arabicToDecimal(String number) {
    char[] chars = new char[number.length()];
    for(int i=0;i<number.length();i++) {
        char ch = number.charAt(i);
        if (ch >= 0x0660 && ch <= 0x0669)
           ch -= 0x0660 - '0';
        else if (ch >= 0x06f0 && ch <= 0x06F9)
           ch -= 0x06f0 - '0';
        chars[i] = ch;
    }
    return new String(chars);
}

prints

42

The reason for using the strings as a lookup is that other characters such as . - , would be left as is. In fact a decimal number would be unchanged.