Confirming based on Gert's comment.
@Gert,
Next time you want to confirm the bug, please simply mark it as confirmed.
** Changed in: gthumb (Ubuntu)
Sourcepackagename: gthumb => language-pack-gnome-de-base
Status: Unconfirmed => Confirmed
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"Leave Fullscreen" not correctly translated.
https://launchpad.net/bugs/55317