Merge local translations instead of keeping only the first one

--use-first removes messages without msgctxt if there's a matching
message with msgctxt. This way all messages without context in our
branch which got a context in master were lost. The downside is that we
might get some fuzzy translations. Those need to be resolved manually,
e.g. by removing them from our local l10n files.

(cherry picked from commit 5660de6ab657c3d0760a3cb7c723c8dd5512ffe4)

• GnuPG-bug-id: T7388