Papers of the masters translated to English in one location Surely someone has collected these papers and translations and has them in a single location for download? For music there is musipedia. Surely there is a mathepedia equivalent?
If I search gauss euler riemann jacobi I get nothing of significance.
If people post answers to sources (that are not already posted) I will collect them all in one resource that can be downloaded and post a link as the answer.
As an update I've downloaded most, except the Euler Archive, of the papers in the links given. This was 168 pdf's/etc.
 A: There are sourcebooks which collect translations of (excerpts from) multiple mathematicians:

*

*D.E. Smith, ed., A Sourcebook in Mathematics (Dover, 1959).


*D.J. Struik, ed., A Sourcebook in Mathematics, 1200-1800 (Princeton University Press, 1969)


*John Fauvel and Jeremy Gray, eds., The History of Mathematics, A Reader (MacMillan Press, 1987)


*Ronald S. Calinger, ed., Classics of Mathematics  (Prentice Hall, 1995)


*Jacqueline Stedall, ed., Mathematics Emerging: A Sourcebook 1540-1900 (Oxford University Press, 2008)
These are all useful for teaching and good for browsing, though none is comprehensive.
They may not have much Jacobi for you, and Struik’s book may end too early for your taste, but I think the rest all have bits of Euler, Gauss and Riemann.
A: Quite a few of Euler's papers are available in translation here: https://arxiv.org/search/?query=euler_l&searchtype=author&source=header
In the original, along with many translations, they are mostly here: http://eulerarchive.maa.org/
I am not sure they are all there.
A: As the many answers already make clear, there is not one dedicated source that collects or archives the work of all the masters. Of course, from your question you get a vague understanding what you consider the masters and the time frame you have in mind. Euler, Gauss, Jacobi, Lagrange, Galois, Cayley, Jordan etc are all masters, but the discussion focused a lot on pre-19th century mathematics. I guess most would consider Neumann, Hilbert and Gödel masters as well.
But where you draw the line? For example, would you include translations of Yuri Matiyasevich work on Hilbert's 10th problems (originally published in Russian)?
My point is that having the synopsis to include all the masters is not well-defined, apart from a collection of a bunch of people it gets blurry for others. Should your collection include the work of Otto Hölder, for example? I have not seen him mentioned here, or in any of the linked answers. What about Georg Cantor? Dénes Kőnig? Kuratowski? Paul Erdös? Grothendieck? They all have papers written not in English.
Ok, sorry for my ramblings. To contribute anything to your original question I have to mention the one the masters considered their master: Euclid. His Elements is available in many translations, see the links at the end of the wikipedia page.
Furthermore, I often come across single papers that are foundational or were written by, what would I consider, a master. To give a list of some, probably less know, examples:
Two Papers by Kuratowski:

*

*Sur l'Opération A de l'Analysis Situs (in French), see also researchgate.

*Sur la notion d'ensemble fini
A translation of the original paper written by M. Presburger showing that the first order theory of the natural numbers is decidable.
A translation of G. S. Makanin's 1966 Ph.D. thesis "On the Identity Problem for Finitely Presented Groups and Semigroups" (2021). This work showed the decidability of equations in free groups (the algorithm actually yields decidability in free monoids). EDIT: This is actually not correct, see the comment by Carl-Fredrik Nyberg Brodda for a more detailed history..
I found a translation of the original paper stating and proving the Sylow Theorems from Sylow.
Translation of a paper by Frobenius.
Translation of a paper by Hölder (unfortunately behind a paywall).
Translation of an often cited paper on synchronizing automata originally published in Slovak by Černý.
Papers (or here) by Axel Thue who worked on patters in strings.
Lastly, Learning via Primary Historical Sources does not contain translations of individual classical papers, but documents that help in reading some of them, even for papers that were originally not published in English.
A: Riemann's collected works (much shorter than most great mathematicians, because of his early death) are published in German https://www.cambridge.org/core/books/bernard-riemanns-gesammelte-mathematische-werke-und-wissenschaftlicher-nachlass/EAB8B46090AFADB656CE9E5969F0A03B.
Here are the original papers in German: https://www.emis.de/classics/Riemann/index.html
There is famous translation of one paper in Spivak's Comprehensive Introduction to Differential Geometry.
Here is Riemann's famous Ueber die Anzahl der Primzahlen unter einer gegebenen Grösse, a paper on prime numbers: https://www.claymath.org/publications/riemanns-1859-manuscript, with an English translation.
A: David Delphenich's site http://www.neo-classical-physics.info/index.html provides "over 20 books and 450 articles on topics in classical mathematics and physics translated from French, German, Italian, and Russian."
The site especially includes translations 19th and 20th century French and German papers and books on algebra, analysis, geometry and topology, as well as translations of physics papers on theoretical mechanics, space-time structure, etc. It has a topic-based selection of papers by a wide range of authors.
A: Here are Wayback Machine links to my English translations, all seven of which are related to Kuratowski's closure-complement theorem.
Sur l'Opération Ā de l'Analysis Situs by C. Kuratowski (1922, in French)
Quelques Notions Fondamentales de l'Analysis Situs au Point du Vue de l'Algèbre de la Logique by M. Zarycki (1927, in French)
General Properties of Cantorian Coherences by M. Zarycki (1928, in German)
Some Properties of the Derived Set Operation in Abstract Spaces by M. Zarycki (1947, in Ukrainian)
On Kuratowski's Problem by V. Soltan (1980, in Russian)
Problems of Kuratowski Type by V. Soltan (1982, in Russian)
Kuratowski Numbers by A. Chagrov (1982, in Russian)
To commemorate the hundredth year since the publication of Kuratowski's paper above, six weeks ago I uploaded And Boundary Makes Three: The Closure-Complement-Boundary Theorem in Topological Spaces to arXiv, page 41 of which gives the following graphical summary of the first century of literature related to the closure-complement theorem.

A: More and more old journals are available online.  There are also excellent machine translators, such as DeepL.
The problem, of course, is that the old journals have been scanned, so the text is not machine-readable.
However, the technology that was used in the past for printing mathematics was, compared to \LaTeX, very simple, as described in my answer and references.  Therefore, as I suggested there, the modifications needed to an optical character recognition program such as Tesseract would be quite modest.
A: Most of the famous 19th century mathematicians have their collected works published and some of them have been digitized. But they are all in the original language. Translations into English are rare. And sometimes they are of poor quality. For example, this translation of Riemann collected papers, or recent translations of Klein and Fricke.
Few works of Gauss have been translated into English.
His diaries are translated in
Expositiones Mathematicae, 2 (2) (1984) 97–130,
but not freely available.
Concerning Euler, there are translation of many mathematical papers, with some of them placed on the arXiv, and there is a
web site which collects his original papers and translations.
Nothing or almost nothing has been translated of Jacobi.
Complete works of Chebyshev are available in Russian and French.
Weierstrass's lectures on elliptic functions are available in German and
French.
There is no single source where you can find all or most of these translations. One has to search for individual books and papers.
High quality translation of mathematical papers is extremely difficult and requires a very high qualification
of the translator and editors. And many mathematicians can read several European languages. So this business is unprofitable, which explains why there are so few translations into English. Those which exist are frequently done by volunteers/amateurs, and are sometimes of poor quality.
Remark. There are many more high quality translations of old mathematics into Russian, which can be explained by certain peculiarities of the Soviet economy, where high quality labor was available at a negligible price.
A: I only wrote this as a comment on Alexandre Eremenko's answer, but after finding more information I thought I would promote it to a whole answer.
Math-Net.Ru is described in the abstract to [1] (arXiv link) as follows:

The main goal of the project Math-Net.Ru is to collect scientific
publications in Russian and Soviet mathematics journals since their
foundation to today and the authors of these publications into a
single database and to provide access to full-text articles for broad
international mathematical community. Leading Russian mathematics
journals have been comprehensively digitized dating back to the first
volumes.

In other words, if you are looking for a paper from a Russian master (e.g. S. I. Adian or A. A. Markov*), then it will likely be there; or, at the very least, its bibliographical information will be.
*The page for A. A. Markov had, until recently, only a handful of articles added. After informing Dmitry Tchebukov, one of the administrators of the site, of this, and sending him a .bib-file with a nearly complete bibliography, he was happy to add these to the site; but not before he had checked each of my entries, and made minor corrections, as well as added a few I had not been able to find! The process seems very rigorous indeed.
[1] Chebukov, Dmitry E.; Izaak, Alexander D.; Misyurina, Olga G.; Pupyrev, Yuri A.; Zhizhchenko, Alexey B., Math-Net.Ru as a digital archive of the Russian mathematical knowledge from the XIX century to today, Carette, Jacques (ed.) et al., Intelligent computer mathematics. MKM, Calculemus, DML, and systems and projects 2013, held as part of CICM 2013, Bath, UK, July 8–12, 2013. Proceedings. Berlin: Springer (ISBN 978-3-642-39319-8/pbk). Lecture Notes in Computer Science 7961. Lecture Notes in Artificial Intelligence, 344-348 (2013). ZBL1390.68748.
