Cuba's "internationalist fighters" have shown the world once again their ability of "march to . . . a far-off place and fight there as if they had been fighting in their own country." President Fidel Castro has proudly reported to his nation.

"We do not want to say things which may appear to be boasting," Castro said in his first public explanation of how Cuba became heavily engaged militarily in its second war in Africa in two years, in Ethiopia's Ogaden region.

But it is not "boasting," Castro said in a televised speech from eastern Cuba Wednesday night, "to say that the Cuban internationalist fighters were characterized by extraordinary effectiveness and magnificent combat qualities . . . That is proletarian internationalism, efficient and brave."

With Cuban troops still present in large numbers on both sides of the volatile African continent, in Angola and now in Ethiopia, Castro's version of what happened in the Ogaden war between Ethiopia and Somalia is very relevant to the past, and also to the uncertain future.

Castro gave no indication this week when the American-estimated force of 12,000 Cubans Ethiopia will withdraw, or what it might do next. Still in Angola, also by American estimate, are 23,000 Cubans - 19,000 military and 4,000 civilian technicians.

According to unofficial and unconfirmed reports, some of the Cuban troops in Ethiopia already have moved to Ethiopia's other large war the secessionist conflict in the northeastern province of Eritrea, where Cuban military advisers previously have been involved.

Cuban spokesmen, in the recent past, have adamantly disclaimed any intention of having Cuban troops enter the Eritrean fighting - where the ideological lines are greatly tangled.

There is virtually nothing in common between Castro's account of Cuba's role in the conflict between Ethiopia and Somalia, and President Carter's denunciation of that activity in his speech yesterday at Winston-Salem, N.C.

TO Carter, Cuba's venture in the Ogaden displayed another "ominous inclination on the part of the Soviet Union to use its military power" in combination with "mercenaries from other communist countries" to "intervene in local conflicts" around the globe.

But to Castro, "The war was a defensive war" for Ethiopia, "absolutely just, to defend the land invaded by foreign agressors" supported by "the imperialists." And the imperialists, in this case, he said, "gave arms to the aggressors - U.S. and NATO arms - through Saudi Arabia, Iran and other countries."

In his lengthy speech, Castro confirmed for the first time that last March he tried to mediate the Ethiopian-Somali conflict, which Somali termed a war to "liberate" nomadic tribes of Somali origin in the Ogaden.

Castro said "we called a meeting in Aden (South Yemen) with the leaders of Ethiopia, Yemen and Somalia . . . to prevent a war, to prevent an act which was treason to the international revolutionary movement, to prevent the Somali government from passing unto the hands of imperialism. It could not be prevented."

Somalia, at that point, was allied with the Soviet Union, which had armed it heavily and which enjoyed strategic naval base rights at Berbera, on the Gulf of Aden. "Somalia had even hoisted the socialist flag," Castro noted. "It was presenting itself as a progressive country . . ."

And Ethiopia, by then, was severing its bonds "with imperialism" as Castro put it, with the United States; power in Ethiopia was in the hands of "the most radical, most important and most revolutionary elements of Ethiopia led by Comrade Mengistu Haile Mariam . . ."

During the Aden meeting last March, Castro said, "the Somali leaders solemnly committed themselves, swearing that they would never invade Ethiopia . . . In reality they had everything planned and in July they carried out the invasion."

Unmentioned by Castro was the Soviet attempt, in early April, by President Nikolai Podgorny similarly to try to mediate, to induce Somalia to join a "socialist federation" with Ethiopia.

(Castro made no mention whatever of the Soviet Union in his account. Even the American-estimated supply of over $1 billion in weapons rushed to Ethiopia by the Soviet Union to repel Somalia after mediation attempts collapsed was described by Castro as simply "socialist supplies" or "modern weapons.")

Initially, Castro said, Cuba decided, at Ethiopian request:

" . . . To send them some dozens of instructors and advisers - perhaps a few hundred - to train their units and to show them how to operate their weapons because, since the emperor [the late Haile Selassie, overthrown earlier in the revolution] was an ally of the United States, they had U.S. weapons and now they were receiving socialist supplies with which they had no experience."

"What determined the need to send fighters?" Castro asked rhetorically, and answered, "The Somali aggression."

Ethiopia, he said, already "had to struggle in many parts of its territory against many groups of counter-revolutionary bandits led by the feudalists with aid from abroad and by secessionist movements in the country's north . . . Time was short . . ."

In mid-December and early January, Castro said, Cuba sent "specialists in tanks, artillery and aviation because, under the circumstances, the Ethiopians did not have time to assimilate the technology."

Although Ethiopia, with a population of 30 million (Somalia has only 3 million people), "did not need troops," Castro continued, "If we sent some Cuban infantry units at battalion level . . . it was a matter of guaranteeing cooperation with the tank and artillery units operated by Cuban personnel."

"The reason was the language problem . . . But actually our fundamental support to Ethiopia was specialists." ". . . in the final phase of the [military] operations," Castro said, "Cuban armored infantry units participated together with the Ethiopian infantry."

The cooperation, Castro said, "was magnificent. There were artillery units composed of Cuban specialists and Ethiopian personnel. In a matter of days, through signs and numbers, they understood each other and the artillery group operated perfectly well . . ."

Somalia's troops, Castro said, "were simply and totally defeated." While they claim to have withdrawn, he said, "If they had stayed . . . just four days more, practically all their troops in Ogaden would have been encircled."

Castro said, "We belive that the war between Somalia and Ethiopia has ended for the present because the territory has been liberated."

He said, "I don't think the Somalis are tempted to commit the stupidity ofo launching another attack against Ethiopia but those who once encouraged them - the reactionary countries, NATO countries and imperialism - can incite them to carry out new aggression against Ethiopia . . .

This indicates that Castro, and Ethiopia, will be in no rush to withdraw Cuban forces form Ethiopia. And meanwhile, Castro raised hope the "the progressive, leftist forces in Somalia" now may be emboldened to return Somalia to the Marxist fold.