German as a distinct language dates back to about the middle of the eighth century. Prior to this the family of Germanic languages (including English) were similar and even mutually comprehensible. Old High German grew out of a dialect known as Western German.
German began to see some stabilization and consistency with the rise of the Holy Roman Empire. The imperial government began using what became known as Middle High German beginning in the 11th century. For centuries strong dialects persisted while the government used a more standard form of the language. In 1522 Martin Luther chose to translate the bible into the standard German used by the government. This version of German became known as New High German.
Until the 19th century, what we know today as standard German was almost entirely a written language and was extremely different from the spoken varieties. It wasn’t until the late 19th century that grammar and pronunciation standards began to appear, and German as a true common language emerged.