West Germany has become the first major American ally to openly set itself apart from Reagan administration efforts to isolate the Salvadoran left internationally, endorsing an effort by German political parties to establish contacts between El Salvador's government and opposition.

The West German initiative was disclosed Wednesday and elaborated on in Bonn yesterday. It is only one of several signs that the U.S. diplomatic drive in Western Europe and Latin America has fallen short of winning the kind of unqualified backing from influential countries in both regions that the administration is seeking.

Britain, France, Italy and even the West Germans called U.S. documents "convincing" when presented by special envoy Lawrence Eagleburger in an effort designed to demonstrate widespread Communist arms support for leftist revolutionaries. The Europeans condemned the Communist support as "unacceptable interference" in the affairs of the violence-torn Central American country.

Behind the scenes, however, leading policy makers in France and West Germany have gone out of their way to stress that their public statements should not imply support for either side to settle the Salvadoran conflict by military means. Their stress on the need for a political solution and for social reforms stands in sharp contrast to the posture of the Reagan administration, which calls for pouring in increased amounts of military hardware, and technical advisers, to assist the government in San Salvador.

The administration's special envoy to Latin America, Gen. Vernon Walters, appears to have been even less successful in winning public support for the administration's first major diplomatic drive. In Mexico, officials responded to the drive with comments praising Cuba and questioning U.S. policy while Walters was making his rounds in the region.

Leading officials in Brazil, while greeting Walters warmly, repeated their opposition to any type of intervention in El Salvador, a measure of the distance traveled since 1965 when Brazil was the only South American country to send troops to assist the U.S. intervention in the Dominican Republic as part of a short-lived "inter-American" peacekeeping force.

Similar sentiments have been expressed by officials in Venezuela and Argentina, two countries generally sympathetic to the government in San Salvador.

In Buenos Aires, Walters stressed that he was making no "request" of the Argentine government and that he was not seeking a possible blockade of Cuba. Argentine officials, nevertheless, were touchy at the suggestion that they might follow the U.S. lead just because Washington beckoned.

"The era of collective decision-making in the Oas [Organization of American States] has ended," said one irritated Foreign Ministry official after Walters left.

The most significant Latin response, and the one which most fully indicates the difficulty the Reagan administration will have in pressing its case, came from Mexico.

In the days since Walters brought his evidence of outside interference to the attention of Mexican officials, Mexican President Jose Lopez Portillo has:

Publicly embraced Cuba as the country in Latin America "most dear" to Mexico.

Warned against the "unscrupulous arrogance of military power" while lamenting that Central America "has unfortunately became elevated to the undesirable rank of strategic frontier."

Warned, on Tuesday, against U.S. intervention in El Salvador, saying it is neither "natural nor reasonable" for powerful outsiders to "fight over our conflicts as though they were the owners."

The West German initiative appears to reflect both strains within the governing Social Democratic Party of Chancellor Helmut Schmidt and a West German tradition of trying to mediate sharp differences between leftists and rightists.

In a public statement Wednesday, government spokesman Kurt Becker said Bonn "expressed its understanding for the American worry over the developments in El Salvador and over the influence of Communist states on the inner conditions of the country."

Becker went on to say, however, that the Bonn government "opposes all use of violence in El Salvador, whether from the right or the left."

Becker said Eagleburger had been informed of the West German decision to try to bring together "the democratic powers" on both sides in El Salvador.

Specifically, Becker said efforts were under way to get representatives of the government junta and the Democratic Revolutionary Front, the left's umbrella grouping to accept invitations from the West German Christian Democratic and Social Democratic parties, respectively, perhaps early in March. The State Department has labeled the Democratic Revolutionary Front a front organization for Marxist-dominated leftist guerrillas.

Sources close to Schmidt said yesterday that while Bonn fully understands U.S. concern about El Salvador, it believes the solution to the country's problems must be political and not military.

French Foreign Minister Jean Francois-Poncet voiced similar sentiments earlier this week when he said at a press conference here that France believes military means will not solve El Salvador's civil strife and that economic and social reforms are required.

British opposition leader Michael Foot, responding to the government's strong endorsement of Washington's demand that outside intervention in El Salvador be stopped, called on Prime Minister Margaret Thatcher to impress on President Reagan during her talks in Washington this week that "many of us would regard it as deeply offensive if such supplies [of additional American military equipment] were to go into El Salvador on the side of tyranny and reaction."