I think you are doing way too much! Just open both files with a zip-like program (I use WinRAR, but most programs will do). You already have
chrome.manifest and
install.rdf edited, so just drag and drop these to the opened xpi, so that you overwrite the existing files with the ones you edited. Then go to the
locale folder in the xpi. This is usually within a jar file in the
chrome folder, but can also be directly in this folder. It is in a jar in this case, so extract this from the xpi and open it with the same zip program. Once in the
locale folder, just drag and drop your complete folder (called
da-DK) from the tar.gz file to the
locale folder to add it to the other locales (after this you can close the tar.gz file). Exit the jar file and drag it to the
chrome folder in the xpi, so that it overwrites the existing jar file. Now you can close the xpi, and you're set for testing!
If it still doesn't work, drop me PM or an e-mail.