Es scheint doch am Gimp zu liegen.
Auf einem Debian, dessen Gimp Deutsch redet:
~$ strace -f -e open,openat,access gimp 2>&1 | grep gimp20.mo
openat(AT_FDCWD, "/usr/share/locale/en_US.UTF-8/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_US.utf8/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_US/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en.UTF-8/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en.utf8/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en/LC_MESSAGES/gimp20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid 1533] openat(AT_FDCWD, "/usr/share/locale/de/LC_MESSAGES/gimp20.mo", O_RDONLY) = 5
~$
Hingegen das problematische Ubuntu:
~$ strace -f -e open,openat,access gimp 2>&1 | grep gimp20.mo
~$
Das heißt, die Sprachdatei wird gar nicht angefaßt
Ist da noch irgendwo zu drehen, oder ist das einen Bugreport wert?