Hi, Canary Bird,
Just reading through this thread again, it occurred to me that you can see most things in two dfferent ways.
In this case, my first view is that you can play anything you like (within reason) on whatever instrument you happen to play.
My second view is that if you want to play music from a specific genre, it would be most appropriate to play it on an instrument that belongs to that genre.
I do both.
Which I do, depends on whether I want to showcase my instrument, on one hand, or the authentic music genre, on the other hand. If I'm just playing for my own pleasure, or to entertain others with pleasant music, I tend to stick to the instruments I can play best, occasionally switching from one to the other, whichever is easier or more appropriate for a particular piece.
(With 5-string banjo, concertina and autoharp, I've got pretty well all bases covered. My public music-making is mostly singing, anyway, and the accompanying instrument is not in the limelight.)
The main thing is that the musical material - the melodic and harmonic character - can be made to work on your instrument. The Richter Scale that is used in the Anglo was, of course, invented in Germany, and is the German concertina's contribution to the Anglo-German configuration. So German folk music fits very well on the Anglo.
Just a bit more authentic in look and sound would be a 20-button German concertina with double reeds, such as Sebastian plays, but the Anglo is not far off!
I play and sing German folk songs for the elderly here, but the instrument I use for that is the German Waldzither - a close relative of the English Guittar and the Portuguese Guitarra, but nevertheless uniquely German. On the other hand, I use the Waldzither to accompany Robert Burns songs, because it so closely resembles the English guittar that was played in polite circles in Scotland in the late 18th century.
At any rate, just do it and have fun!