I don't know much about the early German diaspora, but the topic has always interested me; especially as I'm interested in social/economic stratification, cultural exchange, and historic notions of identity.
Starting from the eleventh century, about one-fourth of the inhabitants of Dalmatia were ethnic Italians, while in may cities (notably Ragusa, modern Dubrovnik) the local ruling class spoke an extinct Dalmatian Romance language similar to (if not intelligible with) northeastern dialects of Italian.
What were the uses of German language in Eastern Europe? Would a well-to-do citizen of Prague, for example, be expected to know German as a citizen of Ragusa would know Italian? When would he speak German, and what sort of accent would he have? Did this change over time?
Was there a language frontier? How far east could a German go and expect to be understood? What sorts of people would be able to understand him?
And finally, following the Russian Revolution, a number of German-Speaking inhabitants of the Baltic States attempted to crate a state run by ethnic Germans in modern Estonia and Latvia. Were there a sufficient number of ethnic Germans living in the Baltic to justify the creation of such a state?
German was widely used in parts, yes. Up until the mid-1800s a good number of residents in cities like Prague and Budapest would have been German-speaking, and cities like Riga and Tallinn (which had the German name Reval until 1918) were mainly German until well into the 20th century.
There were also a good number of German farmers in Ukraine and more east into Russia (the Volga Germans, who had their own autonomous republic in the USSR for a time), brought in by Catherine the Great (my own ancestry included, from the Odessa region). They had a presence up until 1941, when they were largely deported to Central Asia in response to the German invasion. Most ended up in Kazakhstan, where they still constitute about 1% of the total population.