Changing slug for the default language

Hello All,
Happy New Year

I am using WPGlobus and it is working fine.
I have three languages setup and their urls are as following
http://www.domain.com/ (english)
http://www.domain.com/bg/ (bulgarian)
http://www.domain.com/de/ (german)

I want the English default language to also run as following URL
http://www.domain.com/en/

How can I achieve this? Please advise.

Thanks