User1 wrote on User2's wall, How to Localize this one
I'm not sure I understood the question. What would you like to localise?
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User1 wrote on User2's wall,
$Definition['Activity.WallComment.FullHeadline'] = '%3$s wrote on %2$s %5$s.';
$Definition['Activity.WallComment.ProfileHeadline'] = '%3$s wrote:';
$Definition['wall'] = 'wall';
grep is your friend.