What it does:
It will add a language entrance to the customer and order table.
The customer's preferred language can be selected in the create account form or optional be automatic filled in by the selected store language.
This language will be also filled into the order during order process.
In Admin, when an order status update mail is sent, this language will be used in the e-mail independant of the actual selected admin language.
In Admin Orders, the customers language flag is displayed in the order comments label, so you have a reminder which language to use for the order comments.
The customers language can be optional displayed and changed in the customers account and in the admin customers page.
Existing customers accounts will have the language set to 99.
When the customer edits the account profile or when he places an order, the language will be set to the actual store language.