Due to some networking updates, I had to update my FlexSdk3 to the nightly build.
After installing however, the compiler errormessages I got in FlashDevelop suddenly where partially Dutch instead of English.
It was quite a horrible combination and so I needed to change it back to English badly, but unfortunately could find no documentation about it.
After asking on Adobe forums the answer was provided:
Change the jvm.config file and alter the line java.args line to this:
java.args=-Xmx384m -Dsun.io.useCanonCaches=false -Duser.language=en -Duser.country=US