Check if a string is a valid english word, and convert to a valid english word
You should avoid statements on the same line as an if.
So for example  

if (len == 0)
    return true;


instead of  

if (len == 0) return true;


Also, I find that the (s.charAt(0) &lt; 65) || (s.charAt(0) &gt; 90) are really hard to understand as they use magic numbers.

But, most importantly, are you sure your code is responding to all the cases ?

Let's create some unit test with JUnit and AssertJ :

class TestTestString {
    @Test
    void isValidString_shouldAcceptSimpleWord() {
        assertThat(TestString.isValidString("Hello")).isTrue();
    }

    @Test
    void isValidString_shouldAcceptVeryLongWord() {
        assertThat(TestString.isValidString("Thisincrediblylongwordisylbidercnilongz")).isTrue();
    }

    @Test
    void isValidString_shouldAcceptSingleUppercaseString() {
        String[] values = { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N" };
        for (String s : values) {
            assertThat(TestString.isValidString(s)).isTrue();
        }
    }

    @Test
    void isValidString_shouldAcceptStringWithDiacritics() {
        assertThat(TestString.isValidString("Résumé")).isTrue();
    }

    @Test
    void isValidString_shouldRefuseStringThatStartWithALowerCaseLetter() {
        assertThat(TestString.isValidString("apropos")).isFalse();
    }

}


Without much surprise, the isValidString_shouldAcceptStringWithDiacritics is failing... but so is the isValidString_shouldAcceptVeryLongWord because the 120 isn't the good number !
Never forget to do some unit tests :) (you should also add some tests for numbers and special characters)

To conclude, if I were you, I'd get rid of the current test altogether and use the Character methods that are nicer lookings, namely isUpperCase and isLowerCase :

public static boolean isValidString(final String s){
        int len = s.length();

        if (len == 0)
            return true;
        if (!Character.isUpperCase(s.charAt(0)))
            return false;
        for (int i = 1; i &lt; len; i++) {
            if (!Character.isLowerCase(s.charAt(i)))
                return false;
        }

        return true;
    }


Now, if you rerun the tests, they should be all green.

You can also use the same methods for your convert function ;)