@Umherirrender pointed me to the fact, that the translatewiki export script ignores the 3D extension since enabling it months ago.
Output of the repocommit script last night:
--> 3D On branch master Your branch is up-to-date with 'origin/master'. nothing to commit, working directory clean
Today I run export and commit manually:
raymond@web1:~$ php /srv/mediawiki/targets/production/extensions/Translate/scripts/export.php --group=ext-3d --lang='*' --threshold 13 --skip test,aeb,be-x-old,crh,dk,en,fiu-vro,gan,gom,got,hif,kbd,kk,kk-cn,iu,kk-kz,kk-tr,ko-kp,ku,ku-arab,no,ruq,simple,sr,tg,tp,tt,ug,zh,zh-classical,zh-cn,zh-sg,zh-hk,zh-min-nan,zh-mo,zh-my,zh-tw,zh-yue,bbc,ad --target=.
Copying over the result to my own repo on translatewiki server and running repocommit mediawiki-extensions creates the first i18n export: https://gerrit.wikimedia.org/r/390193
I guess that the export script ignores repos starting with a number. Here: 3D