“Ebbinghaus: Language is a system of conventional signs that can be voluntarily produced at any time.
Croce: Language is articulated, limited sound organized for the purpose of expression.
Dittrich: Language is the totality of expressive abilities of individual human beings and animals capable of being understood by at least one other individual.
Eisler: Language is any expression of experiences by a creature with a soul.
B. Erdmann: Language is not a kind of communication of ideas but a kind of thinking: stated or formulated thinking. Language is a tool, and in fact a tool or organ of thinking that is unique to us as human beings.
Forbes: Language is an ordered sequence of words by which a speaker expresses his thoughts with the intention of making them known to a hearer.
J. Harris : Words are the symbols of ideas both general and particular: of the general, primarily, essentially and immediately; of the particular, only secondarily, accidentally and mediately.
Hegel: Language is the act of theoretical intelligence in its true sense, for it is its outward expression.
Jespersen: Language is human activity which has the aim of communicating ideas and emotions.
Jodl: Verbal language is the ability of man to fashion, by means of combined tones and sounds based on a limited numbers of elements, the total stock of his perceptions and conceptions in this natural tone material in such a way that this psychological process is clear and comprehensible to others to its least detail.
Kainz : Language is a structure of signs, with the help of which the representation of ideas and facts may be effected, so that things that are not present, even things that are completely imperceptible to the senses, may be represented.
De Laguna: Speech is the great medium through which human co-operation is brought about.
Marty: Language is any intentional utterance of sounds as a sign of a psychic state.
Pillsbury-Meader: Language is a means or instrument for the communication of thought, including ideas and emotions.
De Saussure: Language is a system of signs expressive of ideas.
Schuchardt. The essence of language lies in communication.
Sapir: Language is a purely human and non-instinctive method of communicating ideas, emotions and desires by means of a system of voluntarily produced symbols.”

Hermann Ebbinghaus, quoted in: Geza Revesz, The Origins and Prehistory of Language, London 1956. footnote p. 126

Source: The Principles of Art (1938), p. 268

Jede Äußerung menschlichen Geisteslebens kann als eine Art der Sprache aufgefaßt werden, und diese Auffassung erschließt nach Art einer wahrhaften Methode überall neue Fragestellungen.
"On Language as Such and on the Language of Man" (1916), translated by E. Jephcott, in Walter Benjamin: Selected Writings, Vol. 1 (1996), p. 62

Michael Halliday (1977). "Ideas about Language" Reprinted in Volume 3 of MAK Halliday's Collected Works. Edited by J.J. Webster. London: Continuum. p113.
1970s and later

"American Indian Grammatical Categories", edited by Morris Swadesh in Word, 2 (1946)
Context: It would be naïve to imagine that any analysis of experience is dependent on pattern expressed in language. Any concept, whether or not it forms part of the system of grammatical categories, can be conveyed in any language. If a notion is lacking in a given series, it implies a different configuration and not a lack of expressive power.

As cited in: Geza Revesz, The Origins and Prehistory of Language, London 1956. footnote pp. 126-127; As cited in: Adam Schaff (1962). Introduction to semantics, p. 313-314
Language (1921)

Chinesisch ist die leichteste Sprache, wenn sie unbefangen gelernt wird, vom Sinn her eher als vom Einzelausdruck. Aber für neugierige Frager bietet die Sprache eitel Tücken.
Die Seele Chinas. Berlin, Hobbing, 1926

Source: 7 March 1942, quoted in Hitler's Table Talk, 1941–1944