Okay, a little diversion into Germanic languages:
broadly speaking, all Germanic languages in the former HRE are West Germanic. We can define three main branches in this:
-Anglic: became English and Scots (before the Norman influences)
-Low German: Dutch (incl. Flemish), Low Saxon (Platt), East Low German (Prussian etc.)
-High German: Standard German, Luxembourgeois, Swiss German, Austro-Bavarian.
Depending on whom you ask, Frysian is either a fourth branch, or a Low German language.
High German is further split in Middle German and Upper German, Low German is further split in Low Saxon, Low Franconian, and East Low German.
Dutch, West Flemish, Limburgs, and Afrikaans are Low Franconian.
"Platt" and Plautdietsch (Mennonite) are Low Saxon
Prussian and Pommerisch (now mostly extinct) are/were East Low German.
Generally and confusingly, all of these are/were called "Platt".
Now on the High German side, standard German (confusingly called "Hochdeutsh"), is a Middle German language. The only other surviving Middle German language is Silesian (the Germanic one, not the Polish one).
A separate group inside Middle German are the West Middle German languages, which are Luxembourgeois, West Middle German (Lotharingisch [but not Alsatian, which is closer to Austro-Bavarian] and Pfalzisch), and Pennsylvania Deutsch.
The other High German languages are the Upper Germanic ones, which means the Swiss dialects, Austro-Bavarian, Swabian, and Yiddish.
With me this far?
Now because people like to be annoying, when a non-linguist speaks about 'High German' (Hochdeutsch), he means Standard German, which is a MIDDLE GERMAN language from the HIGH GERMAN branch of WEST GERMANIC.
When this person speaks about 'Low German' (Niederdeutsch) or 'Flat German' (Plattdeutsch), he means ALL dialects which are not Hochdeutsch. If you've been paying attention, you realize that this includes all LOW GERMAN languages, but also all WEST MIDDLE GERMAN languages (which are not Low German, but High German), as well as all UPPER GERMAN languages (which are also High German).
Now to get back on topic: in the CK era, and VERY broadly speaking, since these languages formed a spectrum, you could say that all Low German speaking peoples understood eachother, and that all High German speakers did. And because the HRE was dominated by High German speakers as Germany is today, the better educated Low German speakers also understood High German.
So to summarize a bit:
Dutch = Low German = Low Saxon = Platt
High German = Standard German
And Frisian is either a close relative of Dutch/LG/LS/Platt, or a closely related cousin.