In German, the corresponding passage (quoted from Effie Briest on Zeno.org) goes as follows:
Zwei der jungen Mädchen – kleine, rundliche Persönchen, zu deren krausem, rotblondem Haar ihre Sommersprossen und ihre gute Laune ganz vorzüglich paßten – waren Töchter des auf Hansa, Skandinavien und Fritz Reuter eingeschworenen Kantors Jahnke, der denn auch, unter Anlehnung an seinen mecklenburgischen Landsmann und Lieblingsdichter und nach dem Vorbilde von Mining und Lining, seinen eigenen Zwillingen die Namen Bertha und Hertha gegeben hatte.
"Fellow countryman" in the English translation leaves out the adjective "mecklenburgisch(en)", i.e. from the Duchy of Mecklenburg-Schwerin, which was succeeded by the Grand Duchy of Mecklenburg-Schwerin (1815–1871). Fritz Reuter was not simply a German novelist: he wrote in Low German (Niederdeutsch), as a consequence of which his work remained virtually unknown in the south of Germany for a long time. An older form of Lower German, Middle Low German, was the lingua franca of the Hanseatic League and influenced the Scandinavian languages.
Mining and Lining Nüßlers are two characters from Fritz Reuter's novel Ut mine Stromtid (1862–64; "From my voluntary-service time"), who make their first appearance in chapter 2 (The novel was translated into High German as Das Leben auf dem Lande ("Life in the coutry side") in 2005. I can easily read Fontane, but for Reuter's work I partially rely on my knowledge of Dutch and I still need to guess a number of words.)
The Hanseatic League had ceased to exist in the late 17th century, i.e. two centuries before the events in the novel.
With regard to Scandinavia: there is an interesting passage in a dialogue between Effi and Jahnke in Chapter XXXIV:
(...); aber zu Jahnke selbst – der nicht bloß ganz Schwedisch-Pommern, sondern auch die Kessiner Gegend als Skandinavisches Vorland ansah und beständig darauf bezügliche Fragen stellte –, zu diesem alten Freunde stand sie besser denn je. (...)
[Effi:] »Ja, freilich schade. Aber auf Rügen bin ich wirklich umhergefahren. Und das wäre so was für Sie gewesen, Jahnke. Denken Sie sich, Arkona mit einem großen Wenden-Lagerplatz, der noch sichtbar sein soll; (...) «
(...)
[Effi:] » (...) Und denken Sie sich, Jahnke, dicht an dem See standen zwei große Opfersteine, blank und noch die Rinnen drin, in denen vordem das Blut ablief. Ich habe von der Zeit an einen Widerwillen gegen die Wenden.«
[Jahnke:] »Ach, gnäd'ge Frau verzeihen. Aber das waren ja keine Wenden. Das mit den Opfersteinen und mit dem Herthasee, das war ja schon wieder viel, viel früher, ganz vor Christum natum; reine Germanen, von denen wir alle abstammen...«
[Effi:] »Versteht sich«, lachte Effi, »von denen wir alle abstammen, die Jahnkes gewiß und vielleicht auch die Briests.«
Translation (from Archive.org) (emphasis added):
But she found a better friend than ever in old Mr. Jahnke himself, who considered not only all of Swedish Pomerania, but also the Kessin region as Scandinavian outposts, and was always asking questions about them. (...)
"Yes, indeed, a pity. Bit I actually did make a tour of Rügen. You would have enjoyed that, Jahnke. Just think, Arcona with its great camping place of the Wends, that is still to be visible. (...)
(...)
" (...) And just think, Jahnke, close by the lake stood two large shining sacrificial stones, with the grooves still showing, in which the blood used to run off. Ever since then I have had an aversion of the Wends."
"Oh, pardon my, gracious Lady, but they were not Wends. The legends of the sacrificial stones and the Hertha Lake go back much, much farther, clear back before the birth of Christ. They were pure Germans, from whom we are all descended."
"Of course," laughed Effi, "from whom we are all descended, the Jahnkes certainly, and perhaps the Briests, too."
The Wends were Slavs, and one of the divisions of the Hanseatic League was known as "Wendish-Saxon" and later as "Wendish and Pomeranian". The chief city of this division was Lübeck.
The Wends were not Germanic, and when Effi mistakenly describes the sacrificial stones as Wendish, Jahnke quickly points out that they are Germanic; he does not want to deny even this gory part of "Germanic history". His fascination for Scandinavia may also be seen in this light: it is purely Germanic.
The most revealing passage with regard to Jahnke's fascination with Scandinavia can be found in Chapter XXIV (in a passage not included in the English translation on Archive.org; see the ellipsis on page 386): Effi tells Jahnke about her trip to Denmark and about her encounter with Thora von Penz:
Thora von Penz, die, wie sie nur sagen könne, »typisch skandinavisch« gewesen sei, blauäugig, flachsen und immer in einer roten Plüschtaille, wobei sich Jahnke verklärte und ein Mal über das andere sagte: »Ja, so sind sie; rein germanisch, viel deutscher als die Deutschen.«
Translation (emphasis added):
Thora von Penz, whom she described as, for lack of a more apt description, "typically Scandinavian", blue-eyed, flaxen-haired and always in a red Plüschtaille, at which Jahnke's face brightened and he said repeatedly, "Yes, that's how they are; purely Germanic, much more German than the Germans."
Jahnke's devotion to Hanseatic League, Scandinavia, and Fritz Reuter appear to be linked through the (Middle) Low German language and a more glorious past for the North-Germanic-speaking world.
