At the Argentinian UAPA website there is an interesting set of pages where for many countries composers and studies are shown.
http://www.problemistasajedrez.com.ar/estudios/compositores-extranjeros/
The Dutch/Holanda article is written by Harold van der Heijden and shows some important Dutch composers and beautiful studies:
http://www.problemistasajedrez.com.ar/estudios/compositores-extranjeros/holanda/
Also there is an interesting article about Jan Timman with 4 unknown studies once published on the website:
"Chess with Masters",
(not in the HHDB6).
The article about Jan Timman with the studies (in Spanish) is written by: Dr. Jesús Seoane Sepúlveda.
http://www.problemistasajedrez.com.ar/wp-content/uploads/2021/08/timman_pdf-1.pdf