Would a computer use du or Sie for the user?
Example: “You have unsaved changes.”
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- 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.
Namehat ungespeicherte Nachrichten. —
Namehas 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.
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, 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. A lot of professional software nowadays does the tedious tasks a secretary or other underling would have done in earlier times, so the boss – even if low in human hierarchy – would probably expect respectful Sie. Other software may be seen more like a colleague, therefore could use friendly du.
It depends on the company making the software in question. For example, Facebook always uses "du", Microsoft always uses "Sie", Apple and Google use a mixture, depending on product and context. Typically "Sie" is preferred in more formal/business contexts, while "du" is preferred in more informal/social contexts.
To give at least a couple of data points, I've looked through the list of most popular apps of 2015. Of these 10 apps, three (Amazon, Skype, Ebay Kleinanzeigen) use "Sie" and seven (Whatsapp, Facebook Messenger, Facebook, Instagram, Spotify, Shpock, Snapchat) use "du", including the three most popular ones.
Also here are the results for the ten most popular websites (in 2013): Six (Bild, Chip, GMX, eBay, Spiegel, Amazon) use "Sie", and four (Wikipedia, YouTube, Facebook, Google) use "du". Interestingly the four most popular ones use "du".
It depends on the usage. A formal usage would prefer Sie, a informal usage would use du.
It is similar to the real life. In a shop and a restaurant I expect a Sie, in a shop or bar for younger and 'cooler' people a du is ok. Same for the computer.
A not so serious classification: Programs in Cobol and Fortran use Sie, Python and Ruby uses du.
Another example: PHPBB offers two language packs: One German (Casual Honorifics) Deutsch (Du) and a German (Formal Honorifics) Deutsch (Sie). A forum for professional usage would prefer Sie, a more private forum uses du. And a good forum let's you decide how the computer should talk to you.
Short answer: if in doubt, "Sie".
As a German software developer, I try to stay clear from "Sie" or "Du". It is perfectly possible to do so without sounding stilted or weird. For example, instead of "klicken Sie hier, um...", it is perfectly fine to use "Hier klicken, um...". It is also a nice excercise to find ways to write stuff more concise to not even have to choose what to do. For example, instead of "Hier klicken, um zu speichern" a simple button with the label "Speichern" is sufficient. This is especially obvious for web sites. I would never have a link labeled "new messages: click me" in a web site. In the same vein, to answer your example, I would not output a message "Sie haben ungelesene Nachrichten", but simply some GUI element which says "ungelesene Nachrichten".
As has been mentioned in the comments, different usages of "Sie" and "Du" pervade German live and it is probably not easily explained to foreigners. For example, some waiters may use "Du" for some customers, while other waiters (at a different location) might use "Sie". In both cases, the other variant may raise eyebrows. There are companies where it would be deadly to use "Du" between colleagues, others where it would be a no-go to use "Sie". Etc.