"Back-channeling" is the English linguistic term for the concept, but 相槌 is actually referred to in normal conversations in Japanese, while most people (the author included it seems) don't know or use it in conversation.

There never seems to be a direct translation into English of compound words, only phrases :/

Would any German speaker be kind enough to tell us what the word for that phenomenon would be? I may or may not write an essay-length article about the unbridgeable differences between the exotic Germanic culture and the Anglo-Saxon worldview based on that untranslatable bit of language.

The techincal term in German is "Rezeptionspartikel", a special kind of "Gesprächspartikel". Wikipedia describes them as follows:

"Sie werden parallel zur Äußerung eines anderen Sprechers oder direkt im Anschluss daran hervorgebracht. Sie stellen dabei das Rederecht des Sprechenden nicht infrage. Beispiele sind hm, hmhm, mhm, ja ..." ("They are uttered parallel to or directly following the statement of another speaker. They do not question the speaker's right to speak. Examples are hm, hmhm, mhm, ja ...")

Source: https://de.wikipedia.org/wiki/Partikel_(Grammatik)#Gespr%C3%... (in German)

Isn't "active listening" an open compound word?