How do I handle a "fake" co-author? I recently started my first postdoc, after completing my phd in a different field. I managed to come up with some results that I hope to publish at a reputable conference.  All the work, including the creative/theoretical part, as well as the programming part was done by me. My plan was to have me and my advisor as the authors of the paper. However my advisor insists that a colleague from a different university, who contributed nothing to the work and does not even work in the exact same field should be added as a third author, with the task of writing the introduction and related work section. I have no idea what my advisor's motivation for this is.

While I feel adding a random person as an author unfairly dilutes my contributions (especially since my advisor is already essentially a courtesy co-author) I could live with being pragmatic about the situation and submit the paper with a third author, even though I feel that writing opening sections to a paper without being involved in the actual scientific process does not justify authorship.

The issue is that person is completely incapable of grasping my work. They understand neither my theoretical arguments nor what my experiments do (and more importantly do not) show. This is at least partially due to the fact they lack a working knowledge of things like probability theory (on a very basic level, e.g. what is a random variable?) that I use in my work.
Worst of all they are not capable of writing somewhat precise English which means that nothing they write is actually usable. I frankly have no idea how they managed to obtain a phd.

I believe that to have a realistic chance to have my work accepted I need to write the sections that were assigned to the third author myself (which I don't mind doing). But how do I handle this situation socially? I feel like now that this person is officially on the author list I cannot simply delete and rewrite what they wrote before submitting, i.e. even though I consider them a "fake" co-author they are now a co-author that needs to clear the final version of the paper.
Hmmm. Socially:

If you have already agreed to it then you should just go along, graciously. In particular, don't take any action that makes vindictive people (who have power) want to sabotage your future. It is only one paper. There will be others. A paper is a good thing, even under such circumstances. If the advisor has some standing, that might actually be good for you. 

Ethically: 

Try to work to disassociate yourself from these people (leeches?) going forward. They are acting in their own interest, not yours. Advisors shouldn't play such a game. Find more honest and ethical collaborators.