Honzuki no Gekokujou: Shisho ni Naru Tame ni wa Shudan o Erandeiraremasen—Dai 2-bu: Hon no Tamenara Miko ni Naru! - Vol. 4 Ch. 20 - Gifts and the Arts

[MangaDex]

 

[Seraphus]

Harfe is German for Harp
Spiel is used as a suffix to denote a natural note scale instrument, such as a Glockenspiel.
That combination of a harp and guitar is popularly played (or was) by a musician named "John Doan" and the instrument (which is a real instrument!) is called...

A Harpenspiel

Harfe+spiel=harfe-spiel, or, in German, after a compounding:
Harfenspiel.

 

[SDsign]

Spoiler

Ah, so this is why Ferdinand and Myne get together, instead of Myne and Lutz. Sad, but definitely better than being with a random noble that cares nothing for her.