The Greek that is translated as “justified by faith” or similar in English is translated in the German translation of Martin Luther (first edition 1522, last revised edition 2017) as gerecht wird (…) allein durch den Glauben: “justified by faith alone” (highlight added).
Luther expained his decision to add allein (“alone”) on pure linguistic grounds rather than as an attempt to emphasize justification by faith:
“I knew very well that the word solum [Latin = alone, only, solely] is not in the Greek or Latin text of Romans 3:28. (…) It’s a fact that these four letters ‘sola’ are not there (…) [But] it belongs there if the translation into German is to be clear and lucid. I wanted to speak German, not Latin or Greek, since it was German I had undertaken to speak in the translation. But it’s the nature of our German language that in speaking of two things, one of which is affirmed and the other denied, we use the word ‘solum’ (allein) along with the word ‘nicht’ (not) or ‘kein’ (no).”
Original text in German
“So habe ich hier in Röm. 3,28 sehr wohl gewußt, daß im lateinischen und griechischen Text das Wort »solum« nicht stehet (…) Wahr ists, diese vier Buchstaben ‘sola’ stehen nicht drinnen. (…) Die Absicht des Textes [ist] gleichwohl »sola« und wo mans klar und deutlich verdeutschen will, so gehöret es hinein. Denn ich habe deutsch, nicht lateinisch noch griechisch reden wollen, da ich mir beim Übersetzen deutsch zu reden vorgenommen hatte. Das ist aber die Art unserer deutschen Sprache: wenn sie von zwei Dingen redet, deren man eines bejaht und das andere verneint, so gebraucht man das Wort ‘solum’ = ‘allein’ (nur) neben dem Wort ‘nicht’ oder ‘kein’.” (source )
Other German Bible translations, including the Zürcher Bibel (which was first published just a few years after Luther’s initial publication) show that the linguistic argument alone is not sufficient. It translates in its current edition: Gerecht wird ein Mensch durch den Glauben — “A person is justified by faith.” The only other German translation that uses allein is Hoffnung für alle (publ. 1983), the German pendant of the English Living Bible.
A Lutheran Low German translation by Johannes Jessen (publ. 1933, republ. 2006) also follows Luther’s translation (alleen dörch Glowen — “by faith alone”).
The only major English translation that adopts Luther’s reading is the Good News Bible (publ. 1976 and often revised) that reads “only through faith.”
See also Luther’s Translation of “Faith Alone” in Romans 3:28 .