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.

• GnuPG-bug-id: T7388