When user creates a account and logs in for the first time with out selecting any option from language or skin then default language and skin will load in email interface .Defaults can be set by admin from manage skins and manage languages in administration panel

but if the user selects some skin or lanague upon log in that will be saved for that user and next time user logs in without selcting any option for skin or lanague there last selection will load

this is also true if they change the skin after they have logged in and then change the skin from the controls present on left side