They're generally interchangeable, but "hard" in this context is broader and hints more at the amount of effort required than the set of skills and abilities you need to possess. For example, digging a garden is not really difficult (since it's easy to learn, and doesn't require any superhuman strength), but is pretty hard (since it takes a lot of time and leaves you tired). You can almost always use "hard" in place of "difficult", but not always the other way. If you find English difficult (ie. it extends your learning capabilities), you will also find it hard (since it will require a lot of effort from you to learn it properly).

When talking about difficulty, hard is an interchangeable synonym. Difficulty is the basic concept and hard is just a shorter word of referring to it. But hard also has other meanings: diamond is the hardest natural mineral; I fell on the floor and hit my head hard.

Personally I would say there is no significant difference. Whichever you use to express your opinion, like "English is hard" or "English is difficult", both of them will allow whoever you're telling this to to understand English is not easy.

It really depends on context sometimes interchangeable. Do you have an example?