Joseph is ost likely Emperor Joseph II of Austria, reigning from 1765 to 1790.

A quick and dirty translation (waiting for improvements):

I'd love to be the Emperor,
would shake the Orient.
The Muslims would have to tremble
Constantinople would be mine.

I'd love to be the Emperor,
Athens and Sparta should become,
like Rome, the queen of the Earth,
the old (times) should be revived (or: what old is will be renewed).

I'd love to be the Emperor,
I'd hire the best of the poets
to sing the deeds of the heroes,
I'd introduce the Golden Age

I'd love to be the Emperor,
but since Joseph pledges his life (? or: during his lifetime will...)
to fulfill my wish
and the wise men are looking forward to that
he may as well be Emperor forever.

Wolfgang