German is the official language in Germany, Austria and Switzerland, also known as the D-A-CH or simply DACH region. Other countries where German is a majority or minority language include Italy (South-Tyrol), Liechtenstein and Belgium.
After Russian, German is the second most spoken native language in Europe with a total of around 95 million speakers.
Differences and similarities
Overall, the biggest differences are in colloquial German where you can find completely different terms for very basic things like food and the syntax can be very different, too.
While the spoken/colloquial language may vary by country or even region, there is a Standard High German which is the official written language in all of those countries and used in all print and audiovisual media. In Austria and Switzerland there are a few alterations to the standard spelling and grammar rules used in Germany but these are minimal. One example is the German ß not being used in Switzerland but replaced with two s.
Translate, transcreate or localize?
For any kind of more formal content a translation into Standard High German will work for all three DACH markets and the smaller ones, too. Examples include: AGB, technical documentation, scientific articles etc.
For more general marketing copy, a monolingual localization by a native Swiss or Austrian proofreader will be enough to correct any spelling variations for these countries. Examples include product descriptions, catalogues, newsletters, etc.
However, the more Swiss or Austrian a product should feel, the more you need to localize or even transcreate the copy. So, this applies mostly to marketing copy or adverts.