If a user shares a link to
example.com/de/about on Facebook, Google+, etc. then the meta text should be in the original language, German.
However, if someone using an English browser clicks on it, it could redirect to the English page, but that can have some problems; see the N.B.
Then again, if they are sharing it on Facebook to their friends or their private circles on Google+, you can assume that many peoples' friends may speak the same language as them anyway. So in this case, doing nothing isn't a problem (i.e. browser language sniffing might not be necessary).
But in the case of public posts (probably, most commonly they'll be Tweets), you may want the links to be relevant to whoever comes across them.
So my answer would be that it depends on what your requirements and users are likely to do — share them privately to their friends, or publicly.
If privately, it might not be worth development time at the moment; though definitely something to look into if it crops up in your analytics that
/en/ pages are often visited by German browsers.
N.B. User language sniffing can be problematic for SEO:
On that note, Google seems to advise against this kind of redirection:
To make all of your site's content more crawlable, avoid automatic redirections based on the user's perceived language. These redirections could prevent users (and search engines) from viewing all the versions of your site.
Putting a visible navigation
English | German somewhere might be better; and is suggested by Google:
In other words, you can provide links between pages with the same content in different languages. This can also be very helpful to your users.
In either case, as per Google's suggestion and my UX reasoning, I would make that navigation visible on every page. (If someone with a browser in German wants to see the English page — why would I stop them?)