I have an unfinished translation of a speciality book. Should I write this down on my CV/SOP?
Unfortunately, most hiring committees would at best ignore this contribution and at worst wonder if you allocate your time wisely.

The work of translators is (unfortunately) given little value in academic communities with the exception of the field of literature and even there it is deprecated. The rationale is that the work is not the result of independent research nor does it add any intellectual value -- it is inherently wholly derivative.

There might be some association of value if your translation had been finished, published by a reputable scientific publisher in your country, and you had been given awards or otherwise acknowledged for providing an important service to your community.  But even then, there is a perception of someone as being "just a translator" that is hard to shake off.

Note that Famous People® are allowed to do translations and be recognized for it, but Famous People® have latitude in their work that non-famous people don't.

At the very worst, a committee might worry that the fact that the translation is unfinished and that you have put effort into doing something with little recognized value means that you don't allocate your energy and time wisely, and that you are not capable of independent research. 

The same also applies for authoring textbooks. Textbooks are also seen as inherently derivative and while they make the publisher and author money, are not indications of original scholarship. 

At this point in your career, you want to be seen as the generator of original scholarship.