Children of a copy-pasted grid element do not update to the correct the sys_languag_uid
Let's assume we have content element (child) within a grid element (parent).
Both elements are of the default language and thus feature the sys_languag_uid "0". Manually copying this parent grid element to a translation page (sys_languag_uid = 1) works just fine for the main element and will correctly update the parent to the new id "1". However any nested child element will still assume the original language id, in this case "0", thus will be missing in the backend and frontend due to its invalid position.
Manually overwriting the sys_language_uid of the grid element child(ren) to "1" makes it all work