Write SQL query to solve given problem: Among the countries that have GNP greater than 1500, what is the percentage of the countries have English as its language?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(IIF(T2.Language = 'English', 1, 0)) AS REAL) * 100 / COUNT(T1.Code) FROM Country AS T1 INNER JOIN CountryLanguage AS T2 ON T1.Code = T2.CountryCode WHERE T1.GNP > 1500