[TASK] Keep existing keys in xliff files when merging

Releases: 6.2
Resolves: #57688

Change-Id: I4f6a5e0b3177add9db67766d1ca2877a2e23fccf
Reviewed-on: https://review.typo3.org/29636
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen