Do I become responsible for debt of my girlfriend when I marry her if she has a debt from her previous marriage. Do I need legal protection?

You do not become responsible for your girlfriend's debt when you marry her if she has debt from her previous marriage.  She is responsible for her debt, and you are responsible for your debt.  If you jointly incur new debt after you get married, you will both be responsible for that debt.  Unless one of your girlfiend's creditors is suing you to try to collect her debt, it does not appear that you need any legal protection.