*TL;DR We have no way of telling but using haplogroup markers (inaccurate and not very scientific), nowadays Poland was most likely relatively devoid of Germanic people, Czech Republic had some sizeable Germanic and Celtic people, and we cannot tell very much about Prussia or Mecklenburg because of how long Germanic people hold it after conquering it from Slavs, making these numbers only useful to tell how many Slavs were there.

The longer version:

It's difficult to know for sure. The ratio of the original Germanic people still living there during the Slavic migration would be difficult to know as well, as Germanic and Slavic tribes were often attacking each other taking women from one another, trading and sometimes intermarrying, making the genetic makeup of both countries weird. However if we are to go for the "black and white" resolution that would in theory define the ratio, we have to assume that certain haplogroups actually represent Slavs and Germanic people. The main contender for a 'Slavic origin' male gene is the R1a haplogroup (and its subsequent [NOT ALL] mutations), and the R1b haplogroup for the Germanic origin.

NOTE: The thing is, R1b is highly likely of Celtic origin (given its spread) while actual proto-Germanic origin is what the majority is in Scandinavia (I1), meaning that the proto-Germanic tribes that came in contact with Gauls and other Celtic people were absorbed by the overwhelming numbers of Celts, especially during the era of Frankish Kingdom.

Now this assumption uses the logic of "the most frequent gene is the original one" so this is sort of unconfirmed pseudo-science but not completely dismissable and has at least a little bit of basis that can cast some light on this issue. Let's use this site. https://www.eupedia.com/europe/european_y-dna_haplogroups.shtml

Speaking about Poland, its R1a (in our case the "original Slavic"[although it's not correct to claim so]) percentage is over 57%. R1b (the "original Germanic") is around 12,5%. It means that the majority of people were of actual Slavic origin coming from a Slavic male. 12% are of Germanic descent. However it does not mean that these people are actual Germanic people, that person could've had 12 Slavic ancestors and 1 German father effectively making it look like he was of Germanic origin. This is the problem with this school of thought.

Now Czech Republic: 33% R1a, 28% R1b. Czech Republic is a home to many Celtic sites, and given the mountainous terrain around Czech Republic, the Celts were in a good spot to preserve themselves. It is in my opinion the main reason for the high R1b frequency which maintained its numbers when Slavs came (Slavs were assimilating countless tribes when they invaded), while the Germans from the Sudettes most likely hightened the frequency during the HRE.

Prussia no longer exists, however it is the historical land of Baltic tribes that were replaced/assimilated by Slavic tribes, that were Polonized and then Germanized, then back Polonized AND Russified post-WW2. Now this is difficult to tell, but Baltic tribes tend to have a high frequency of the R1a group, which we said should represent the Slavic population (in reality, in Europe it represents the Balto-Slavic branch [linguistic term but can be used in this context] and IE peoples in general). So Prussia (the Eastern part) was most likely overwhelmingly R1a-ish.

Now when it comes to Vorpommern and Mecklenburg, two Westernmost Slavic areas during Charlemagne, they were settled but we do not know if heavily. After the Franks took over those parts of lands, they defeated the armies and assimilated a large portion of remaining Slavs. This can be seen once again on the site https://www.eupedia.com/europe/european_y-dna_haplogroups.shtml. It shows us that Northern and Eastern Germany have over 22 and 24% of R1a haplogroup, while it's 36% for R1b in those same areas, meaning that the Slavic males were relatively abundant there.

Now Thessaloniki and Northern Greece overall were known to be invaded by Slavs and that the Slavs did not go away, staying there and effectively becoming Greek overtime. We can actually see this in the haplogroup marker of Northern Greece, with 18% being R1a, the second highest one, right after 20% E1b1b which is Balkanic/Mediterranean marker.