View Reference Values in pick list field not showing

View Reference Values in pick list field not showing

Dears,
I want to create a custom field, type "Pick List", to define the language of a Lead.
I want a pick list with some options value ("en", "fr", "de", etc.) and I want to display this options with a reference ("English" for "en", "Français" for "fr", etc.). 
I know that I should use Reference Values for this, but in my case I cannot find the link to that :



Can someone help me?