I created 2 languages for my site English and French, I also created dictionary items named "Login_Username" and "Login_Password".

I created a user control, in the code behind I have

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!this.IsPostBack)
            {
                this.lbUsername.Text = library.GetDictionaryItem("Login_Username");
                this.lbPassword.Text = library.GetDictionaryItem("Login_Password");
            }
        }

I created macro by using this ASCX control and inserted it into one of my node.

I navigated to http://localhost:8901/authentication/login.aspx?umbdebugshowtrace=true which is a content node with user control macro embedded, at the bottom of trace data I saw 

library Error returning dictionary item 'Login_Username'
Object reference not set to an instance of an object.
  at umbraco.library.GetDictionaryItem(String Key)  

library Error returning dictionary item 'Login_Password'
Object reference not set to an instance of an object.
  at umbraco.library.GetDictionaryItem(String Key)

What is wrong here?