how to find i18n_variables

I have tried to translate title field label in my custom content type editing page (admin/content/types/). Title is labeled now as "Item name" but it should be translatable to other languages as well.

I noticed that there's a handy way to use i18n variables to translate some submitted text. Like this:

 $conf['i18n_variables'] = array(
  'site_name',
  'site_slogan',
  'site_mission',
  'site_footer',
  'site_frontpage', 
 );

Only problem is that I can't find the name of the variable I should use. If I look content_types.inc I can see that "title_label" is related to this field but it doesn't work :-(

 $conf['i18n_variables'] = array(
  'title_label', 
 );

I suppose it's too generic. What is the correct variable name - or is this solution working at all?

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