You don’t need Profile 2 for this. You can add a field to the form from Core.
|field_perfered_language||List (text)||Select list||edit||delete|
I added this to the Manage Fields from here:
Home » Administration » Configuration » People » Account settings
This gives the user a drop down choice with the two languages. With English as the default. This does not affect the language that is chosen when the account is enabled but at least you know what they want.