Well, on the other hand, the greatest musician after the Greeks composed the majority of his greatest creations only after he stopped hearing German.

Well played, points for cerebral humor.

In the meantime, weren't there some very important European scientists, mathematicians, philosophers, etc who wrote almost entirely in Latin?

Some people blame the tragedy of 20th century on prevailing idealistic trends in German philosophy, including the ones you mentioned.

Santayana's Egotism in German Philosophy is an admirable and very readable history and diagnosis of that story, from Leibniz to the late 19th C. He has a gorgeous style and lovely understated sense of humour. And virtually explains the Nazis, although published in 1915.

There were certainly plenty of great German speaking physicists and mathematicians. But those philosophers all sucked.

'The greatest' ?

Slight exaggeration.

With a few minor exceptions, such as Aquinas, Descartes, Voltaire, Locke, Hume, J.S. Mill, Russel..