The quote is from a speech Rudio gave at the Town Hall in Zürich on the 6th December 1883; The German original is published in Felix Stähelin, *Reden und Vorträge* (1956, I have not found it online).

An English translation of the full speech is here. The translated quote reads as follows:

But I cannot move on from reviewing Euler's mathematical work without
having considered an important factor. I have said that mathematics is
a language in which natural phenomena can be described in the simplest
and most comprehensive manner. With this in mind, you will understand
how important it is to express mathematical thoughts themselves as
concisely and clearly as possible. In this respect, Euler's work was
epoch-making. We can be safe to say that the whole form of modern
mathematical thinking has been created by Euler. If you read any
author immediately before Euler, it is very difficult indeed to
understand his terminology, as he has not yet learned how to let the
formulas speak for themselves. This art was not taught until Euler
came along.

speak for itself?;-) $\endgroup$ – Francois Ziegler Mar 14 at 16:38