Does a senior area editor supervise a country or a region?
Most journals accept papers from a fairly large spectrum within some field. Most of the time the field contains specialties that require deep knowledge. No person can be expected to be expert in all of those sub fields. Therefore, a journal's editorial staff is likely to contain people with at least some knowledge of a specific subfield. For a math journal there may be "area editors" for Algebra, and Analysis, and such. Perhaps even sub-sub fields within those fields. 

So "area" here refers to area of expertise, not a geographical designation. (Possibly different in a journal devoted to geography, I suppose.)

But even here, the specific knowledge of an editor is limited, hence the assignment to reviewers who are expected to be much more knowledgeable in the specific of a given paper. The area expert (editor) can do a rough estimation of the quality and suitability of a paper, but it requires deeper analysis. 

A journal that published in various languages, of course, would need people to sort those, but that isn't the main idea behind the term.