Custom Back Button for Navigation Bar

I made a category of UINavigationBar as follow:

UINavigationBar (UINavBar_Category)

I want to create custom back button using this.

I can able to set image by following code:

[[UIBarButtonItem appearance] setBackButtonBackgroundImage:[UIImage imageNamed:@"btn_back.png"] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];

Problem can not able to change title of back button.

My try

 - (void)didMoveToSuperview
{
UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithTitle:lang(@"btnBack") style:UIBarButtonItemStylePlain target:nil action:nil];
    self.topItem.backBarButtonItem=backButton;
}

Help me to solve this!

Thanks in advance