Comment # 5 on bug 1039305 from Dr. Werner Fink

Both version do accpet the denglish version of dekodieren in the large version

 ispell -d german
 @(#) International Ispell Version 3.3.02 12 Jun 2005
 word: decodieren
 ok

 word: 
 igerman98/igerman98-20160407> ispell -d deutsch 
 @(#) International Ispell Version 3.3.02 12 Jun 2005
 word: decodieren
 ok

 word:

and even Duden does know it

 http://www.duden.de/suchen/dudenonline/decodieren

 Von Duden empfohlene Schreibung: decodieren
 Alternative Schreibung: dekodieren
 Worttrennung: de|co|die|ren, de|ko|die|ren

... it seems that some denglish words are already part of the native language
german.