What are the best german chess books never translated to English?

I know about some books written by Tarrasch, like Die moderne Schachpartie, but I would like to hear about some other classics that might be escaping my attention.

I guess the variations must be outdated, but what about the explanations?

There are many German (as well as Spanish) books that were originally written in Russian (and then translated into German and Spanish). Keres, Romanowsky, Koblenz, Bondarewsky, Boleslavsky, and Suetin spring to mind. Quite simply, most of those are worthwhile because those Soviet players were excellent coaches and/or didacts. The writing style may feel dated (and sometimes have a political flavour) compared to today's breezy and streety argot, but if you ask me, I prefer that old dry as dust way of writing, because they have so much great knowledge to impart.

That said, Keres was an excellent annotator, a world class player, and therefore his explanations can be fairly relied upon, although the variations themselves may not be quite up to scratch anymore. This is worth bearing in mind, since there are 'renowned theoreticians/opening experts' who are/were not world-class players. Their understanding is likely to be inferior to that of the world-class players. This was particularly case in the past with theoreticians like Krause, Møller, Svenonius, Schwarz, and a number of other well-known names. They may have been able to spout variations until the cows came home, but they lacked the sophisticated understanding of players like Tarrasch, Lasker, Alekhine etc. all the way up to Kasparov. Since then silicon monsters have become a great leveller (not that differences still don't exist, but they're much less noticeable).

Would you mind naming a few of those russian books that have been translated to Spanish and are not available in English? I'm interested, since my native language happens to be Spanish.

I can't say for sure they've never been translated into English, but at least they're not very well-known in their English editions (although Quality Chess is now making up for lost time), while any well-sorted second-hand chess book seller would have these in Spanish. I'm including a couple of more books by Richard Réti and Bent Larsen, respectively, which to my knowledge have not been published in English editions:

Alexander Koblenz (the spelling varies Koblentz, Koblencs), Tal's trainer as I'm sure you know, wrote a couple of really good books on chess training and tactics: "Ajedrez de entrenamiento", "El Dominio del Arte de la Combinacion" and "El mundo magico de la combinaciones".
Igor Bondarevsky: "Ataques directos al rey" and "Tactica del Medio Juego" (although I think the latter has been translated into English as well).
Akiba Rubinstein: "La partida de ajedrez" (thought to be the only book exclusively written by Rubinstein)
Richard Réti: "Curso científico de ajedrez" and "Estudios completos"
Bent Larsen: "Teoria y practica de los jogos abiertos"
Paul Keres: "Manual de Aperturas Abiertas"
Paul Keres & Iivo Nei: "25 x 4" (25 games each by Fischer, Larsen, Spassky and Korchnoi)
Vasili Panov: "Teoria de aperturas, tomo I abiertas y semi-abiertas" and "Teoria de aperturas, tomo II cerradas" (although Panov is one of those noted theoreticians, which were less successful as players).

Thanks! I'm going to check those books.

Hmm...The best chess books are in the Czech republic.

But english chess books are good, too.

Aus meiner bescheidenen Bibliothek kann ich folgendes für die englische Übersetzung vorschlagen:

Diese Denkmäler des Schachs könnten auch sein:

Aber vor allem dieses Denkmal für Schach und Schachliteratur, das ist:

Ah! Entschuldigung:

https://www.chess.com/forum/view/chess-equipment/stupidity-in-chess-books-publishing

Das Schacherbe Aljechins - Alexander Kotov. 

Euthyphro399, this is a very good suggestion by noblestone (I had forgotten about this one, despite having it myself!). Published in Spanish as a four volume work worked entitled 'Herencia ajedrecistica de A.A. Alekhine' or 'Herencia de Alekhine'.

Thank you all for the new recommendations.

I have a very high regard for Alekhine, but I am already in possession of his two-volume game collection (annotated by himself), so I'm not so sure about how much new material I would be able to find in Kotov's books. Since it is a four volume work, perhaps it contains a lot of games that are unfamiliar to me, or it might be case that Kotov offers some new insights into formerly studied games.

The book entitled "Dawid Janowsky (1868-1927)" has caught my attention. This is explained by the fact that I'm used to study games on which Janowsky is thoroughly defeated. But I assume that he must have been an interesting player after all, and that I might have come across a very unfortunate selection of his games.

In my view, Kotov was a great author, and what he does in 'Herencia de Alekhine' is that he's looking into how Alekhine was breaking new ground and how he influenced those who came after him (for instance, Alekhine was virtually the first player to consistently prepare openings for specific opponents, and to conduct more independent and thoroughgoing opening analysis).

David Janowsky was a fine player, although not even close to world champion class (Lasker, Capablanca and Alekhine toyed with him like a cat with a mouse). He was far to impetuous and stubborn, and threw away many games by excessive confidence in his own powers and his extreme reluctance to agree draws. That said, he was a dangerous attacking player, and handled the bishop pair beautifully.

The famous Bilguier Handbuch was not translated. And, as far as I know, some of Tartakower's books on hypermodern chess have not been translated.

Although I am not 100% sure they were never translated:

1. Französisch Winawer by Kindermann (the best book ever on the Winawer)

2. Theorie der Schacheröffnungen Teil VIII (French and Caro-Kann) by Euwe (you will find a lot of interesting insights and variations here although the are not top-notch anymore of course)

3. Die Tschigorin-Verteidigung by Morosewitsch (this one was translated but is much cheaper in german)

Generally the newer books are almost always better in english or they are only published in english.

Hope that helped!

Klauer: Colditz was completely unknown to me (it's on my 'To buy' list!). Kurt Richter was well-known for us older chessers in Sweden (the Kurzgeschichten um Schachfiguren was published as Schackkavalkad [Chess Cavalcade]). I really enjoy Richter's writing style, highly inspirational!

It seems to me like a very interesting recommendation. I'm going to try to acquire it in the future. 
For the moment, I'm more interested in game collections, but I think that it would be useful to diversify the type of books that I use.

This is a nice book; Sternstunden des Schachs - 30 × Olympia :