为en-US和en_US创建区域设置有什么区别？

I am having all my resourcebundle values in table and formatted as per requirement.i have to change the languages in the website based on User selection in drop down in top of the page. If i use language code as en_US then its working fine. if i Use en-Us as Language Code then its not working. What might be the problem. Which is correct to follow?I am having all my resourcebundle values in tab