Store the language choice in the Session, a Cookie, or a DB entry associated with that particular user. For non-web based, use the DB, or store it in the application settings.
Then in your Page Load or Form Create code you check the required language, and off you go.
I'm assuming this is WinForms - in which case it's probably easiest to use the application settings:
Managing Application Settings (.NET)[
^] will explain how to use them.