Associating current German dialects and ancient tribes is hackwork from the 19th century. The language we call German now is the language Martin Luther used to translate the bible while in Saxony and under the protection of the Saxon prince-elector. It replaced the local standards since it was more popular in print; The Jesuitendeutsch language, for instance, was removed by decree from Maria Theresia in 1774 because it wasn't used by anyone save Austrian Bureaucrats (and academic poets) at this point. The Hansesprache in the North was also still used in the city councils in the late 19th century (see Buddenbrooks by Thomas Mann), much like the Norman language is in London till our days, but as a language of commerce it was gradually getting replaced by Luther's German since the 16th century.

The modern German language developed from what modern Germanists will call East Central German, from the language of Saxony, and has since taken up a ton of vocab and of phrasing from the other dialects as well.