Note: I've written about this so many times that I've cobbled together most of my older answers to create this more or less comprehensive answer to the question.

The Nazi's policy towards Slavic peoples wasn't as consistent or thoroughly planned out as many people think. Like most other elements of Nazi government policy, their official stance towards Slavs was highly malleable and opportunistic and was changed according to circumstance. If you could write a general summary of how the Nazis viewed the Slavs, it would be something like "Slavs that give us what we want and fight for us are good, Slavs that don't give us what we want, occupy our Lebensraum or fight back against us must be destroyed". The Nazis had no problem with arming and fighting alongside Bulgarians, Croatians and Slovakians, but planned for the destruction of Russians, Czechs, Poles and Ukrainians.

The one thing that you have to completely get out of your head when talking about Nazi government policy is the notion that everything that the Nazis realized (like the Holocaust, their economic programs, the Hunger Plan etc.) before and during the war was part of a well-planned out scheme hatched before 1933. This is something that media depictions of Nazi Germany ALWAYS DO and it always bugs me. In reality, the Nazi government acted on a mostly ad-hoc basis, usually responding to developments in retrospect rather than consciously planning them out. The reason for this was because Nazism, like other fascisms, sold itself as a non-doctrinal ideology. They prided themselves on their non-doctrinality because it was what made them (in their eyes) more vigorous and dynamic than other competing ideologies.

Before the outbreak of war, the Nazis were actually ambivalent towards the Slavs. For instance, figures like Herman Goering admired the Poles for their rejection of Bolshevism and triumph over the Red Army at the gates of Warsaw. He especially had a thing for Marshal Pilsudski, whose translated collected works Goering wrote the forward for. Most Nazis had a general idea of Slavs being vaguely inferior to Germans before the war; a brochure for the 1938 Nuremburg Rally listed Slavs alongside Germans and Celts as one of the 'Indo-Germanic peoples'. Eventually, Ukrainians, Cossacks and even Russians were allowed to fight under the Germans which they justified by claiming that those ethnic groups had some percentage of Aryan or Germanic blood. When the war started turning in the Soviets' favour, Himmler became convinced that Stalin was actually a lost Aryan-Asiatic tyrant like Genghis Khan.

As the war went on, Nazi Slavic policy became even more convoluted. Figures like Himmler and Hitler became convinced that the 'Slavic race' never actually existed, and that it was a propaganda fabrication of the Tsarist court's pan-Slavists. These Nazis claimed that Poles, Russians and Ukrainians did not exist as unified races like the Germans, but were divided into 'Nordic, Subnordic, Dinarian, Praeslavic and Eastern' racial types, some of whom could be properly 'Germanized'. The main goal of this complete turn around in policy seems to have been the destruction of any national unity in Eastern Europe and the sowing of rivalries between favoured 'Nordic' and 'Subnordic' communities and the other proposed racial types.

So what did the Nazis actual plan to do to the so-called Slavs who they weren't sure really existed? Plans from the Reich-Fuhrer SS dictated that 85% of Poles, 75% of Byelorussians, 65% of Ukranians and 50% of Czechs would have to be deported from German-occupied territory to Western Siberia at the end of the war to make room for German colonists, with the remaining population being forcibly ‘Germanized’ as mentioned above. These deportations may well have been a euphemism for a planned genocide, where the displaced Slavs might have been worked to death in Siberia. Before the war had ended, the Nazis planned to kill as many Slavs through starvation as possible which was actually initiated with the Hungerplan, where all food resources in the East were directed to German military forces. The Hunger Plan was inspired by German food shortages caused by the British blockade in WWI combined with a genocidal plan to kill as many Soviet Slavs as possible to make room for post-war German colonization. Because Soviet POWs were under the direct control of German authorities who also controlled their access to food, they were the largest demographic affected by the Hunger Plan; 3 out of 5 Soviet POWs died in German captivity for the duration of the war, and most died of starvation.

In reality the destruction or removal of the Slavic population was impossible. The amount of Soviet people set for destruction would have left the Soviet east completely unworkable. There were simply not enough German colonists to make the removal of Slavic populations feasible; even during the war German colonists barely made inroads in occupied Poland, and even there they had to rely on local labour. Low German birthrates and available labour would make the wholesale removal of Russian, Ukrainian and Polish populations simply unfeasible. Unlike the Jews, the Slavs weren't really perceived of as a unified race which made their complete destruction impossible as well. Basically, the Nazis had no clear or unified Slavic policy, and when they did develop one, it was basically impossible to actually implement in the real world.

EDIT: and I just realized that you asked about what the Nazis did to Slavs IN Germany. Wow. I'll get back to editing this monstrosity to reflect that.

The treatment of Slavs IN Nazi Germany is something I've never actually read about. It's a very unknown topics, likely because there weren't too many Slavs in Nazi Germany. I can however tell you how they treat Slavic people in occupied areas.

First let me tell you what Nazi Germany's plan for Slavic territories was. He planned to take over the land all the way up to the Ural Mountains, and exterminate/deport all the Slavs gradually, that were living on the land West of the Urals. At first they would be used for slave labor until Germans were able to take care of everything by themselves, then they would be killed or deported past the Urals. Then they would use the European Slavic land for farming and colonizing.

In Poland, Poles were prohibited from having sex with German women, and would be killed for it without a thought. As many Ethnic Poles were killed during World War 2 as Polish Jews, each around 2.75 million.

In Belarus, where Partisans were also active, Germans would regularly slaughter civilians/lock them up in Orthodox Churches or Synagogues and burn them down. There were regular mass slaughters and Belarus lost the highest percentage of their population in the entire war (25%), followed by Poland and Ukraine. Killing Slavs was Hitler's plan for the war. I don't wish to minimize the casualties of Jews, and I never would considering I am both Jewish and Ukrainian. 24 Million Slavic people died during WW2 and around 6 Million Jewish People died. The Germans treated Slavs the same as they treated Jews, worse in some situations, better in some, but generally they were the same. Slavs were the subhuman race defending "Jewish Bolshevism"

Considering the way that Slavs were treated outside of Nazi Germany, any Slavs inside would have been killed without a doubt. I hope I have answered your question.

The ultimate plan for Eastern Europe was outlined Generalplan Ost. The ultimate goal was to expel, enslave, or exterminate the "Untermensch" Slavic population in order to make room for German settlers. It was an outline for carrying out Lebensraum and not fully recognized, but an engineered famine known as the Hunger Plan killed millions in Eastern Europe. Ukraine and Poland were especially decimated under Nazi occupation.

There is a lot to it, but I think one good example is the German occupation of the city of Kharkiv, Ukraine. It lasted from October 1941 to August 1943, with a brief reoccupation by the Soviets for a month in February 1943.

Sources vary, but the city's population was at least 700,000 before the invasion. The 1939 census recorded about 850,000 (Moskoff, 2002). By 1943, the number was down to roughly 200,000.

The city's large Jewish population was targeted for extermination in the first few weeks of the occupation. German settlers were sent in, and they quickly established German-only areas separate from the remaining Slavic Ukrainians (Subtelny, 2009). As time went on 120,000 people were shipped to Germany for forced labor, and another 100,000+ were executed or starved to death. I have read one account (a 1943 CBS report) of bodies being hung from balconies across the main street in the center of the city, but I am unsure how true that is.

When the Nazis retreated in early 1943, they implemented a scorched earth policy and blew up major buildings across Kharkiv. Then, when the Soviets were forced to retreat a second time, the Red Army also blew up buildings and destroyed food supplies (Collingham, 2012).

EDIT: I also misread it as "by" rather than "in." I'll try to add more.

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Works cited:

Collingham, Lizzie. 2012. Taste of War: World War II and the Battle for Food. Penguin Books.

Moskoff, William. 2002. The Bread of Affliction: The Food Supply in the USSR During World War II. Cambridge University Press.

Subtelny, Orest. 2009. Ukraine: A History. University of Toronto Press.