Hi,
You can do it easily.
( you need to study the documentation about "language editing").
Follow the steps given below
1. login as admin
2. Go to "site administration->language->language editing
3. click on "edit words or phrases" (now you will see the files and words used in moodle).
4. select "moodle.php"
5. After that(you still in same page), click the button "switch lang directory"
6. Now you can modify the any words you required( you can find the sentence "you are not logged in", modify it as your requirement).
I'm sure it definitely works
Rate me.
Have a nice day