Works for me: "Aktuelles Bild verkleinern oder vergrößern" is shown (eog-3.16.2 on Fedora 22)

Are other tooltips in the headerbar working for you? Does the tooltip change if you toggle the button?

Yes, other tooltips are translated and if I toggle the zoom button the tooltip does change (and is translated into german too).

Can you try whether

gettext -e -d eog "Shrink or enlarge the current image"

will output the translated string?

What distro do you use?

Oh, I think you can leave the -e out of the command.

Another idea: Is it the same if you start eog with a different locale.
For example: LC_ALL=fr_FR eog -n

Here is the output:

sworddragon@ubuntu:~$ gettext -d eog "Shrink or enlarge the current image"
Shrink or enlarge the current imagesworddragon@ubuntu:~$ 


And with the french localization I'm seeing the same issue.

Edit: I'm on Ubuntu 15.10 dev.

It looks like Ubuntu's shipping the package with some outdated translation.

I took the translation file for eog from the language-pack-gnome-de-base_15.04+20150416_all.deb (Ubuntu apparently ships translations independent from the application) decompiled it and compared it with intltool against the eog-3.16 sources. This is the result:

270 translated messages, 14 fuzzy translations, 4 untranslated messages

while the PO file from eog is fully translated:

288 translated messages

Please ask Ubuntu to ship correct/up-to-date translations.

I have now forwarded this issue to Ubuntu: https://bugs.launchpad.net/ubuntu/+source/eog/+bug/1464848