IntelliJ spellchecker comes with only English and Arabic bundled (strange, I think it is made in east Europe, they didn't even bundle their language?).
My customer is German so all my code is mixed English (code)/German (interface) and I can't find a German dictionary for IntelliJ.
Current IDEA versions load dictionaries in UTF-8, you don't need to convert them to the platform encoding, ignore the iconv step below.
The dictionary can be produced using aspell for Unix/Mac OS X or under Cygwin. You need to have aspell and appropriate dictionary installed.
Here is the example for Russian dictionary I've used:
aspell --lang ru-yeyo dump master | aspell --lang ru expand | tr ' ' '\n' > russian.dic
For German it would be:
aspell --lang de_DE dump master | aspell --lang de_DE expand | tr ' ' '\n' > de.dic