import java.lang.Character.UnicodeBlock;

public boolean isLatinOnly(String s) {
  for (int i = 0; i < s.size(); i++) {
    Character ch = s.charAt(i);
    UnicodeBlock block = UnicodeBlock.of(ch);
    if (!Character.isLetter(ch) || block != UnicodeBlock.BASIC_LATIN ) {
     return false;
    } 
  }
  return true;
}

println("asdf is latin only: ${isLatinOnly("asdf")}")
println("ASDF is latin only: ${isLatinOnly("ASDF")}")
println("asDF is latin only: ${isLatinOnly("asDF")}")
println("!asDF is latin only: ${isLatinOnly("!asDF")}")
println("абфак is latin only: ${isLatinOnly("абфак ")}")

asdf is latin only: true
ASDF is latin only: true
asDF is latin only: true
!asDF is latin only: false
абфак is latin only: false

Set<Character> latinLetters = new HashSet<Character>();
for (char ch = 'A'; ch <= 'z'; ch++) {
 latinLetters.add(ch);
}

for (int i = 0; i < s.length() ; i++) {
 if (!latinLetters.contains(s.charAt(0)) {
  return false;
 }
}