Update: The code below only worked on iOS7.
Like Rikkles said, you don't need to customise the text to make it localized. But just in case someone ever has a good reason to do this, here's the code:
NSArray *activityItems = @[@"Test"];
UIActivityViewController *activityController = [[UIActivityViewController alloc] initWithActivityItems:activityItems applicationActivities:nil];
NSAttributedString *as = [[NSAttributedString alloc] initWithString:@"Custom Text" attributes:@{NSForegroundColorAttributeName:[UIColor greenColor]}];
[[UIButton appearanceWhenContainedIn:[UIActivityViewController class], nil] setAttributedTitle:as forState:UIControlStateNormal];
[self presentViewController:activityController animated:YES completion:nil];
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