The dog-eared book of foreign phrases that monolingual Yankee tourists have toted across Europe for decades is about to be replaced by a computer - a computer that translates.

Punch an English word or phrase into the calculator-like keyboard, tap the translate button, and the machine flashes the appropriate words in Spanish, French, Italian, German or Portuguese.

It costs $225 - with a plug in module for one language - knows 2,200 English words and phrases and goes on sale at Hechts Dec. 15.

Hecht's expects to sell out by Christmas - if the 500 computers the chain is getting last that long.

Made by a Florida firm called Lexicon, the translator is about the size of a paperback book and comes with a multi-national electric plug.

It has a big battery and a tiny brain, a microprocessor, a computer smaller than a matchbox.

Microprocessors are the technological breakthrough that's made possible this season's most gee-whiz Christmas presents - the computer that talks, the television games that play more games than an entire arcade, and the computers they play chess better than all but master players.

Steven Chafitz, president of Chafitz in Rockville, says his company will sell "in excess of $10 million" worth of Boris, a $300 chess playing computer. Chafitz manufactures and distributes Boris, which was created by some engineers who figures on selling a thousand a year.

The makers of the pocket translating machine say they have orders for $3.6 million worth of their product and anticipate selling out their entire production through next March.

The translator-maker, Lexicon Corp. is a year-old public company that spent nearly five years developing its machine, refining the programming and shrinking the electronics into a pocket-siez package.

Plug-in programing modules that teach a new language to the basic keyboard adn controls sell for $65 each. A calculator module is also planned, one that will have special programs for currency transactions. For jet-setters, a new module will offer a few basic phrases in seven language.

There are five languages now, but a total of 13 are planned, including Russian - which will require an overlay to show the characters on the keyboard - and Chinese and Japanese. The calculator-style readouts can't duplicate oriental characters, so the Japanese and Chinese versions will reply phoenetically.

A second generation translator that actually speaks the foreign language is under development and Chafitz is already selling a machine that talks - but only English.

Called Speak & Spell, it's made by Texas instruments and sells for $50. Speak & Spell teaches kids words by pronouncing the word aloud, then repeating the letters aloud as they are punched into the keyboard. Type the wrong letter and the machine politely tells you you're wrong.

Using its microprocessor - not a tape recorder - to do the pronounciation, the machine has program modules that start with dog and cat and work up to words that adults freqently misspell.