I don't think they are compressed, because I already tried to open/extract them with 7zip, all I got was an error because of the incompatibility. The other thing is, that when I open the files with the "Editor", I can see my messages there, but there are many many other symbols there, which makes it impossible to read the messages. They are too wide spread and you can't tell which messages belong together etc.
Even though I have no idea how to "decompile" WhatsApp, I won't ask you to tell me, since it sounds pretty complicated. I hope there is another way to solve this problem.
Yes, I simply saved the file on my Desktop, didn't use the BackUp function.
That's right, they're "just" for backup, and I didn't grab just the IsolatedStorage folder, I took the whole "Data" folder, just like aramadsanar told me to.
Thanks again for your fast and detailed answer.