I wrote a paper intended for a conference in computer science. Before submission I hired an expert to review and edit my paper. Do I need to mention them? If so, in what way? As a collaborator? In a footnote? In a special "Thanks" section? Would it be unethical not to mention them at all?
The answer will depend on the significance of the editing done.
The one extreme would be significant intellectual contributions to the work -- for example, shaping the overall narrative of the paper, making major decisions on the information being emphasized and de-emphasized, and organizing how the information is packaged into different sections and parts. Such editing would qualify for authorship. The ethical thing to do would be to offer authorship to the editor. If she declines, it would be necessary to at least mention her in an acknowledgement.
The other extreme would be sentence-level editing, like correction of typos and grammar quirks. Such editing would surely not quality for authorship, and it would be up to you if you mentioned the editor in the acknowledgement.
The middle ground between both extremes really is somewhat of a gray area.