The German fachidiot does indeed fill a gap in the English lexicon, until today I might have described a highly qualified person with little to no social or practical skills as someone who could have Asperger's syndrome. People who have this condition may be very focussed and experts on one or two subjects, (source) they can become obsessed in a particular field, and seem to live in their own world.
Looking at the German Wikipedia article, my browser translated the following excerpt into comprehensible English, any emphasis is mine.
As specialist idiot (especially in Austria and specialized trottel) a is an expert called that a problem only from the perspective of its subject area knows only from his point of view (narrow-mindedness can grasp) […]. Often the Fachidiot finds it difficult to empathize with others and to supplement its limited view and expand. Further possible ways of looking at it rejects due to selective perception (selfishness , narcissism) and consequent ignorance
The above description shares some similarities with Asperger's syndrome, but labelling a person is a risky affair, and should be avoided unless you are qualified to diagnose such a disorder. In alternative, may I suggest the following:
2. extreme preoccupation with and indulgence of one's feelings, desires, etc.; egoistic self-absorption.
1. narrow-minded and subjective; unwilling to understand another viewpoint.
2. an inordinate or obsessive zeal for or interest in a single thing, idea, subject, or the like.
However, none of the above really fit with the description of a fachidiot, a "subject fool", someone who knows everything about one discipline but nothing about anything else, the Independent article, written in 1998, goes on further to describe the late Margaret Thatcher:
Despite Oxford, the Bar, and her work as an industrial chemist (when she was partly responsible for the invention of Mr Whippy ice cream), she is a woman of painfully limited interests. [...] His [Dennis Thatcher] wife has no recreations or hobbies of any sort. She tried to interest herself in Chinese porcelain, but she remains a complete philistine, and notably ill-read. Her single- mindedness has been a strength, but it takes displeasing forms.
It appears that the German term draws attention to the narrow field of interests rather than the the person's incompetency or foolishness outside their specialized area of expertise. No one would ever say that the ex-British Prime Minister was an idiot, but many would agree that she was myopic, blinkered, and a solipsist.
Lastly, Dict.CC the German English Online Dictionary par excellence, offers the following translations for fachidiot
- geek [esp. Am.] [coll.]
- nerd [coll.] [obsessive expert in a specific industry]
- blinkered expert [Br.]
- one-track expert
Note, there is no mention of idiot, fool or dunce.
All definitions from Dictionary.com