i am good in speaking English and writing , but I never understand foreigners English why ?

This is because of their accent. In other words, in India we have a particular manner of pronouncing a word. That's our accent. But in foreign countries, theor way to pronounce a word is different. Both the languages are English, but the accent or manner of pronouncing is different and that is why it become difficult sometimes to understand what they are saying. For example, Brittish Accent and American accent - both are different. But if you have listen to them carefully you can start understanding what they are saying. You can watch videos of YouTube, don't look at the subtitles, just listen what they are saying. This helps to learn their accents and listen to the words carefully.