Conversation so far:
Why is learning a second language difficult?
Children have a much easier time with the grammar because they learn in a different way than adults do
At what age is learning a second language harder and why?
Evidence indicates it becomes harder to learn a language other than our mother tongue as we progress through adulthood. Now, a new study suggests that children are highly skilled at learning the grammar of a new language up until the age of 17 or 18, much longer than previously thought

Question: If I already know English which language would be the easiest to learn
Self-contained rewrite: If I already know English, which language would be the easiest to learn?