The expression "interested to work" isn't quite right; it should be "interested in working". That would mean to have an interest in working, or in this case, being open to working at that place. (You can sometimes use "interested to", mostly with verbs of perception like "see", "read" or "know". For example, as in "I'll be interested to find out who won the competition". That means that you will find the result of interest, and is slightly different in its emphasis from being "interested in finding out", which is more about the process of discovery.)

I make an ad offering a job. And the job is somewhere different from the location of the newspaper or website. eg a job in Spain, listed on a British website. I don't want you to apply for the job if you're not interested in working in Spain. It would be a waste of time. So I ask you this question.

It's asking you whether you would be willing to work at the location mentioned (probably in an ad, article, etc.). By location it means probably the city closest to where that job takes place.