impractical = nicht machbar
unpractical = unpraktisch
So sah das bei mir bisher vereinfacht aus.

impractical
adjective
1. Not adapted for use or action; not sensible or realistic: impractical high heels his impractical romanticism

1.1(Of a person) not skilled or interested in doing practical work: Paul was impractical and dreamy

2. chiefly North American Impossible to do; impracticable.

http://www.oxforddictionaries.com/definition/...

unpractical
adjective
Another term for impractical (sense 1 - see above)

http://www.oxforddictionaries.com/definition/...

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For me, these are synonyms. For what it's worth, I don't recall ever having heard or seen impractical used in the sense given in 2. If anything, I'd expect to see impossible or (rarely) impracticable in that case.

OED also has the following comment on the difference between impractical and impracticable:

Although there is considerable overlap, impracticable and impractical are not used in exactly the same way. Impracticable means ‘impossible to carry out’ and is normally used of a specific procedure or course of action, as in "poor visibility made the task difficult, even impracticable". Impractical, on the other hand, tends to be used in more general senses, often to mean simply ‘unrealistic’ or ‘not sensible’, as in "in windy weather an umbrella is impractical".

Da schau her, ein Amerikanismus. Normalerweise habe ich das immer andersrum. Es ist bei mir aber auch denkbar, dass ich den Unterschied zwischen "impractical" und "impracticable" bisher immer überlesen habe.

There was some discussion of this by native English speakers in this Crossover Chat thread (from post #174 onwards): Siehe auch: Crossover Chat *140* - #174

And here: Siehe auch: Crossover Chat *141* - #11

"Impractical" is common, "unpractical" is very uncommon. People might say "not practical," but rarely "unpractical."

https://books.google.com/ngrams/graph?content...