EDIT: to clarify, the Unification of Germany in 1871, not the German Reunification of 1990.
Language and cultural/regional identity seem to be quite strongly connected, so I was wondering if the Unification of Germany had any consequences on the German language. Since this is still a very broad question, I'll try to break it down into sub-questions:
Did the people of different regions consider themselves to be speaking the same language before the unification? Wikipedia gives the impression everyone already spoke "German" before, but did people actually call it "German", or was it more like how French, Italian and Spanish are all "Latin" languages? If it's the latter, how did it transition to German as we know it now? If the perception was more as different dialects, were they notably affected? And what happened to non-Germanic languages spoken in the region?
I apologize in advance as I'm on my phone. I'll post sources in a little while.
Roughly speaking, the West Germanic dialects on the continent exist in a dialect continuum starting from the Netherlands and Belgium and going roughly southeast until Austria and the German communities in the former Austria-Hungary area. As such, there is a wide range of dialects in Germany. Someone speaking their local German in the north of Germany today would have trouble understanding the local dialects of Bavaria and Austria. But most urban people speak Standard German, or Hochdeutsch, albeit locally accented. This wasn't always the case, but much like Standard Italian the language became popular as a means of communicating with such a broad range of dialects. At least in the north, the local dialects eventually gave way to speaking Hochdeutsch. Local dialects are still very common in the south and are featured in the middle of the country, but Hochdeutsch is still popular.
Hochdeutsch is a High German dialect. I mentioned earlier the dialects spoken in the north largely gave way to Hochdeutsch. Back in the day, these areas spoke Low German dialects. Low German isn't very closely related to High German. I believe it's more closely related to English and Dutch, but don't quote me on that. The dialects spoken in the central and southern parts of Germany are also High German dialects. A lot of these dialects are more or less mutually intelligible with Hochdeutsch. Thus, those speaking Low German in the north had an incentive to learn and start using Hochdeutsch. Unfortunately, this had the effect of greatly reducing the number of Low German speakers (let alone the individual dialects).
For some sources, Wikipedia is pretty good and I have actually submitted questions to both /r/AskHistorians and /r/linguistics on this subject:
http://www.reddit.com/r/linguistics/comments/1r2bam/what_led_to_the_development_of_northern_germany/
Did the people of different regions consider themselves to be speaking the same language before the unification?
If it's the latter, how did it transition to German as we know it now?
There were several written languages across the first Reich. The Hanse had it's Hanse Dutch, the Catholics did have their Jesuit Dutch; but the language of Luther was the one most used in print because it could be read both in the North and in the South. Maria Theresia swapped the Jesuit Dutch state language of Austria for Luther Dutch in 1774 because that was the language of the educated books. The Dutch themselves have called their own language *Nether-*Dutch in the time before all the Germans chose German
By the 1840s all educated people in Germany certainly had one common written language but the common pronunciation wouldn't be developed until the end of the century so that the poetry of Mozart, Heine, Goethe and Schiller still rhymed best in their respective native dialects.