>* Du hast ungespeicherte Nachrichten. — You have unsaved messages.
>* Sie haben ungespeicherte Nachrichten. — You have unsaved messages.

Both occur, but often neither is used, because it’s usually possible to phrase a dialog or message in an impersonal way without becoming too distant, e.g. passive voice or _man_.

* _`Name` hat ungespeicherte Nachrichten. — `Name` has unsaved messages._
* _Ich habe ungespeicherte Nachrichten. — I have unsaved messages._ 
* _Es gibt ungespeicherte Nachrichten. — There are unsaved messages._
* _`##` ungespeicherte Nachrichten — `##` unsaved messages_
* _ungespeicherte Nachrichten: `##`  — unsaved messages: `##`_
* _`##` Nachrichten nicht gespeichert. — `##` messages not saved._
* _`##` Nachrichten noch nicht gespeichert. — `##` messages not saved yet._
* _`##` Nachrichten wurden noch nicht gespeichert. — `##` messages have not been saved yet._
* _Sollen alle Nachrichten gespeichert werden? — Should all messages be saved?_
* _Nachrichten speichern? — Save messages?_
* _Alle Nachrichten sollen gespeichert werden! — All messages should be saved!_
* _Alle Nachrichten speichern! — Save all messages!_
* _Speichere alle Nachrichten! — Save all messages!_

Some of the examples above turn the original informative statement into an actionable option, dialog or command. Which one of these is most appropriate, depends a lot on the situation and local user interface guidelines, which may apply to other languages as well. That would be more on topic [elsewhere](http://ux.stackexchange.com), though.

Sometimes, e.g. in video games, the software has a persona and the user (or player) does, too. Their (simulated) social relationship determines the level of familiarity, respect and politeness, hence the appropriate pronoun.