In the face of denunciations by 12 other Latin American countries for its recognition of leftist guerrillas as a "representative political force" in El Salvador, Mexico's Foreign Minister Jorge Castaneda said today that he expects some Latin American nations and "European countries of great prestige" to come out in support of the Mexican position, in which France has joined, within the next few days.

Castaneda said that "this is not the first time or the last that Mexico has found itself relatively isolated" from other Latin American states. He cited Mexico's refusal to break relations with Cuba during the 1960s as an example.

The joint French-Mexican declaration, issued a week ago, called for a negotiated solution to the Salvadoran civil war, and by giving major international recognition to the Salvadoran guerrillas precipitated a virtual diplomatic war.

Both the governing military-civilian coalition in El Salvador and the United States, which is supporting it, have rejected any proposal to negotiate the composition of the current government. They prefer to seek a political solution through elections next year, although the guerrillas and their political allies are not expected to participate.

The Mexican-French document calls for the will of the Salvadoran people to be "expressed through authentically free elections." But it calls first for "a process of of global political solution, by which a new internal order would be established, the armed forces would be restructured, and the necessary conditions created for the respect of the popular will."

A dozen Latin American countries, including Venezuela, Colombia and Argentina, have condemned the Mexican-French initiative as intervention in El Salvador's affairs.

Although the Mexican-French declaration was clearly a move toward international legitimacy for the Salvadoran insurgents, Castaneda said there was no intervention because no coercion is involved.

In a prepared statement he emphasized that the declaration issued a week ago "does not recognize the Salvadoran opposition as a legitimate government or as a belligerent" in the juridical sense.

Asked flatly whether Mexico favors the Salvadoran guerrillas, Castaneda said that he could not answer categorically, but that Mexico frequently has criticized "the action of El Salvador's government junta for the state terrorism it is developing, for the violence, for the repression, for the crimes that they have committed."

Castaneda was careful to say he does not think Mexico's diplomatic support of the Salvadoran guerrillas will seriously affect relations with Washington or the planned Cancun summit conference of 22 heads of state, including President Reagan.