Following on from the changes in PR #4781, we can make
additional message strings translatable using the
tr.Tr.Get() method.
Because this method performs printf(3)-style format string
parsing and interpolation, we can simplify some of the
surrounding calls, e.g., from fmt.Errorf() to errors.New(),
and from fmt.Fprintf() to fmt.Fprintln(). This ensures
that if either the translated text or any interpolated
arguments happen to contain character sequences that would
be interpreted as Go format specifiers (e.g., "%s" or "%d"),
these will not result in warnings such as "%!s(MISSING)"
in the output text.
Note also that we try to remove newlines from the message
strings were possible and change the surrounding calls to
append them instead, e.g., with fmt.Fprintln().
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