After all, there are probably thousands of middling poets who express regret and helplessness.

German:

Also, Infanterie Greift An by Erwin Rommel is a fantastic memoir that I reviewed here on the site a bit earlier.

I would assume Der Weg Zurück would also count, or even Drei Kameraden if you take a long view of the war. Of course, by that standard you could take Hesse's Der Steppenwolf or Mann's Der Zauberberg or Döblin's Berlin Alexanderplatz or even Musil's Der Mann ohne Eigenschaften.

Zweig's Die Welt von Gestern is a great framing work.

English:

Of course, I would expect that just opening the Fussell book will give you a long list of English-language sources (Fussell was terrible about German sources - could he even read German?). Hemingway's A Farewell to Arms covers the Italian Front pretty well.

Lawrence's The Seven Pillars of Wisdom is probably also a must-read.

Russian:

There really aren't a lot of good Russian works on World War I, because the Revolution and Civil War that followed get so much more press. There are parts of some books written by authors who lived through the period which discusses the "war before the Revolution" somewhat, like Pasternak's Доктор Живаго and Sholokhov's Тихий Дон, and a few poets seem to have written some words about the war, like Anna Akhmatova, Marina Tsvetaeva, Alexander Blok and Sergey Esenin, but there is hardly any direct assessment made, and most works about the period are completely devoid of information about what happened before 1917.

French:

I actually don't know of anything about French war literature.