Files are loaded in a particular order (alpha sorted, within a given path location) and when multiple copies of the same file are loaded, whichever was loaded last stays. So, if you wanted to override a file in data-1.1.0.pk3, you could make another pk3 file with those modified files in it (using the same names and paths within the pk3/zip file as were in data-1.1.0). Then, put your new pk3 in the same place as data-1.1.0.pk3 and name it anything that's alpha-sorted after data-1.1.0.pk3. Perhaps you'd go for data-german.pk3 or something.
That said, as David said, much of the text isn't actually in the .menu files and is instead hard-coded, so there's that problem on top of your not being able to use a (modded) pk3 on pure servers without those servers having the same pk3 installed (though they can make having that pk3 optional for their users). Currently the game isn't written in a way that's particularly conducive to translations. Sorry
