Learning French, Arabic, Italian, and more -- at the same time

''Monolinguals are dinosaurs.'' That is a contention Lee Riethmiller can argue fluently - in French, Italian, Portuguese, Russian, Spanish, German, Japanese, Greek, Arabic, or Turkish. In a year or two he expects he will add to the list Amharic, the language of Ethiopia. He has also set his sights on Chinese and Hindi.

Mr. Riethmiller, who teaches 27 classes in nine languages in a rented room in the Cambridge YMCA, says there is nothing extraordinary about people who speak several languages, other than that they have overcome their fear of languages - and developed a discipline similar to that of the marathon runner.

To prove his point, he's designed a course in which he teaches six languages at once. It is a method he says is the wave of the future. This is partly because of his conviction that in a shrinking world a working knowledge of many languages will be mandatory.

''Monolinguals'' are a disappearing species, he tells a high school audience. ''You are a transitional generation that is walking away unchained from a monolingual past into a multilingual earth. And you are going to have to decide if you are going to become a victim or a beneficiary of this.''

The most difficult part for the novice, he says, is not in learning the words , but in learning how to learn languages.

It is the class called Quinta-Lingual I (Spanish, French, Portuguese, German, Italian, and Arabic) that Riethmiller says is breaking new ground. The once-a-week class, which began in January, has nine students - including two computer programmers, a registered nurse, a business analyst, and a medical student.

The languages are taught with emphasis on their unique rhythms, intonations, and even gestures to allow the students to hear and feel the differences. The students are expected to perceive each of the languages as separate ''personalities'' to avoid confusing, for example, Italian with Portuguese.

Riethmiller says the language personalities permit students to switch from each language as one switches television channels. The students say in moving from Italian to Arabic to Spanish, for example, they are forced not to rely on English to make the transition. They must make the connection in Italian, Arabic , and Spanish.

His students could have a working knowledge of all six languages in three years, Riethmiller says.