See tracker for details. it appears, according to pfl, that gimp installs locale files to /usr/lib/locale instead of /usr/share/locale.
Could it be false-positive? I will recheck for gimp-2.10-36 and gimp-2.99.18 but currently installed gimp-2.10.38 has locale under "/usr/share/locale/": $ equery f gimp | grep locale /usr/share/locale /usr/share/locale/ru /usr/share/locale/ru/LC_MESSAGES /usr/share/locale/ru/LC_MESSAGES/gimp20-libgimp.mo /usr/share/locale/ru/LC_MESSAGES/gimp20-python.mo /usr/share/locale/ru/LC_MESSAGES/gimp20-script-fu.mo /usr/share/locale/ru/LC_MESSAGES/gimp20-std-plug-ins.mo /usr/share/locale/ru/LC_MESSAGES/gimp20.mo and non files under "/usr/lib/" (empty output of `$ equery f gimp | grep "/usr/lib/"`).
Let me check myself now...
I can't reproduce it either! Sorry for the noise.