Assuming your using extended POSIX, you were very nearly there with the first example posted: The following seems to work for me:
https?:\/\/[^/]+?\/[^/]+/en/
If you want to capture the 2nd path element, then the following sed extended POSIX regex also works
sed -E -e "s/https?:\/\/[^/]+?\/[^/]+\/([^/]+)\/.*/\1/"