When and where does the short story "The Interlopers" take place?

The short answer to your question can be found by reading the very first sentence in the story:  “In a forest of mixed growth somewhere on the eastern spurs of the Karpathians, a man stood one winter night watching and listening.”  The Carpathian Mountains stretch from the Czech Republic to Romania in Eastern Europe, and its eastern spurs would most likely place this forest somewhere in either Ukraine or Romania.

This particular “narrow strip of precipitous woodland” is on the outskirts of the forestlands owned by Ulrich von Gradwitz, on the border of the family’s property.  The land had been given to his grandfather in court, taken from the Znaeym family, the neighboring landowners who had never acquiesced to the ruling of the court but had instead continued to illegally hunt on this bit of forest, which they considered to be rightfully theirs.  Over the generations a blood feud developed, and now Ulrich von Gradwitz and George Znaeym are mortal enemies.  When we are introduced to Ulrich, he is in the contested land with his rifle waiting for Znaeym family raiders.

As far as when this story takes place, this also is answered in the first sentence:  we are in winter.  A “wind-scourged winter night,” to be specific, two generations after the initial contestation of the border forest, and therefore two generations deep into the rivalry between the Gradwitz and Znaeym families.