I'm mostly interested in the Holy Roman Empire around the time of Martin Luther, but I'd love to know information about ethnic politics within earlier or later periods of the Holy Roman Empire.
Did ethnicity matter much within this context, or were all non-Jews in the Holy Roman Empire treated basically the same despite their origins? Negative stereotypes of Poles and Italians are quite prevalent in Europe today, but did any noticeable chauvinism exist among German-speakers in the 16th century?
I’ll have to split this response into bits, as this this is a lot of history-
Initially, the Empire was split into three kingdoms: German, Italian, and Burgundian. They were basically administered as though they were the same place, although the Emperor found it most convenient to spend all of his time in Germany. Another thing to note here is that a pre-requisite to becoming Holy Roman Emperor at this time was being King of Germany, which meant that Emperors only needed to curry favour with a small part of their Empire. Consider at this stage that Slavic areas (then including not only modern day Poland and Czechia but the old East Germany) were outside the Empire, and the most significant ethnic troubles they faced were from the restive Saxons. This system worked relatively well until around the 1000’s.
The 11th/12th century brought with it-
The Investiture Wars, which was fought to work out the position the Church (or more accurately the Pope) held within the Empire and his rank compared to that of the Emperor.
The Lombard League, which attempted to break the Kingdom of Italy free from the HRE, which eventually failed.
This as well as several other more minor war changed the way the Kingdoms were considered in the Empire. The Investiture Wars somewhat brought on the use of King of the Romans as heir apparent instead of King of Germany, and French involvement in Burgundy as well as the Lombard Wars brought an effective end to the “kingdoms” as they were. However, this was offset by an expansion east into the “Saxon Marches” (East Germany), Pomerania, Silesia, and Bohemia, which found itself as a kingdom. The former three rather quickly became ethnically German, which they all remained until 1945. Bohemia had a more special status within the Empire, as the only King and Prince at the same time, it was awarded considerable autonomy. The former Kingdom of Italy wavered in and out of Imperial control in this time, most obviously in the case of Venice, which manage to get out completely. Cities in Italy mostly ran themselves, only using the Emperor as a font for honours and advantages to use on their enemies. However, sometimes the Italians received a prompt message in the form of the furor teutonicus, an old Latin phrase reused in a new context. However, things would soon change with new reforms coming in the latter half of the 15th century.
When the Habsburg came into power, they embarked on three major reforms- the establishment of the Kreis (or circles), which were administrative groups that were not used outside of Germany, the Reichskammericht, a Supreme Court that only had jurisdiction in Germany, and the Reichshofrat, a court handling “imperial prerogatives”, which had jurisdiction all over the Empire. At this stage, the ethnic minorities went through one of 3 fates-
(Burgundy) getting swallowed into a larger ethnic union, and entirely leaving the Empire.
(Italy) either setting themselves free entirely, or being so independent that they might as well be.
(Bohemia) Being stuck inside the Empire (in Bohemia’s case, they were inherited by the Habsburgs) and keeping their old rights from way back when they were a small Slavic principality.
The Poles were pretty much out of the picture at this point, having gone the way of the Sorbs and the Wends as Slavs who had been subsumed into Germany.