Users who typed "nicer" into Reverso looking for a French equivalent were offered the example: "Hitler was a lot nicer to the Jews than they deserved."
A search of "much nicer" produced the result "Dachau was much nicer than Auschwitz."
The French-based service used by more than 45 million people a month threw up equally hateful results for the word "Jew".
"There are too many Jews here", "Here is the ultimate example of how the Jews control America" and "This is why the Jews are so dangerous" it offered as examples of how the word is used.
AFP found that translating "Jew" into Italian brought up, "We will knock on the door of the mosques with Jewish skulls", while a search in German on "Muslim" threw up the result, "A good Muslim always keeps his mouth shut."
The International League against Racism and Anti-Semitism, known by its French acronym Licra, condemned Reverso, saying its "examples of translations were dripping in anti-Semitism".
It called for immediate action to fix the problem.
The site's founder Theo Hoffenberg told AFP that its machine translating tools had never been known to slip up so badly.
- 'Unfortunate and horrible' -
"We have never had such shocking examples that need such rapid correction," he added.
Hoffenberg promised to correct the "unfortunate and horrible" examples pointed out by Licra within hours and said the site would be carrying out a "complete revision of all potentially risky terms" and words.
He said the billions of examples of how words can be translated in its "Context" feature were managed by algorithms based on common expressions and problems set by linguists.
Reverso uses subtitles from films and internet videos to replicate the most commonly used phrases from natural speech.
Hoffenberg said the site is not corrected in real time but every few weeks based on feedback and recommendations from "several thousand" of its users.
Jew and Muslim are not the only words that throw up questionable results.
A search done by AFP on the French word for "blacks" gave the English translation "Known fact -- blacks move in, crime goes up", "To be fair, most animals hate black people".
A search in English gave "He was shot by two blacks outside a garage."
The site previously got into hot water in 2015 for offering up pornographic explanations for certain German words.