How can a ghost travel in different countries?

Spirit lives are human, despite earthly ideas to the contrary. 

In other words, the mistakes of the earth have to be put right here.

How does one get to another country here?

In several ways.

Shanks’s mare is one of them.

There is only one kind of barrier you will come across in this world, and that is the barrier between one realm and another, and that is invisible—or practically so. 

It is a natural law that works in this way, and like all such laws, it works without any breakdown, fuss—or trouble.