Restore missing translation for the downstream theme.

Upstream changed between Havana and Icehouse, and one of the strings we
were relying on downstream is not present anymore in the upstream
message catalogues. Restore the string for Japanese, based on the
previous translations.

Resolves rhbz: 1122785

Change-Id: Ie0a9df9cb54585fa63e2fab4786dee62ffb6d1e3