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 with **Asperger's syndrome**. People who have this condition may  be very focussed and experts on one or two subjects, ([source][1]) they can become obsessed in a particular field, and seem to live in their own world. 

However, labelling a person with a specific condition is a risky affair unless you are qualified to diagnose such a disorder. In alternative, may I suggest the following:

>**[solipsist][2]**   
>2. extreme preoccupation with and indulgence of one's feelings, desires, etc.; egoistic self-absorption.

<nbrsp></nbrsp>  
>**[blinkered][3]**  
1. narrow-minded and subjective; unwilling to understand another viewpoint.

<nbrsp></nbrsp>  

>**[monomaniac][4]**        
2. an inordinate or obsessive zeal for or interest in a single thing, idea, subject, or the like.

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*][5], the article in *The Independent*, written in 1998, goes on 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 seems 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**, and **single-minded**.


All definitions from *Dictionary.com*


  [1]: https://en.wikipedia.org/wiki/Asperger_syndrome#Restricted_and_repetitive_interests_and_behavior
  [2]: http://www.dictionary.com/browse/solipsist
  [3]: http://www.dictionary.com/browse/blinkered?s=t
  [4]: http://www.dictionary.com/browse/monomaniac
  [5]: http://www.independent.co.uk/voices/comment-my-name-is-margaret-and-im-a-recovering-prime-minister-1180641.html