Goethe wrote it in German: “Wer fremde Sprachen nicht kennt, weiß nichts von seiner eigenen.” It first appeared among the aphorisms in his journal Über Kunst und Altertum (vol. 3, no. 1, 1821) and was later collected in his Maxims and Reflections. It means you only really understand your own language once you can compare it with another.
What it means
Someone who speaks only one language takes it for granted and cannot see how it works. A foreign language gives you something to compare it with, and shows you your own grammar, idioms and habits of thought.
Where it comes from
The maxim was published among the short aphorisms in Goethe's periodical Über Kunst und Altertum (On Art and Antiquity), third volume, first issue, 1821. These aphorisms were later gathered, with others, into the collection known as Maximen und Reflexionen (Maxims and Reflections). The German reads: “Wer fremde Sprachen nicht kennt, weiß nichts von seiner eigenen.”
The popular English is a close, standard translation. Bailey Saunders's English Maxims and Reflections (Project Gutenberg reproduces Macmillan's 1906 printing) renders it as no. 414, in the section “Literature and Art”: “A man who has no acquaintance with foreign languages knows nothing of his own.”