A group of Democratic senators yesterday asked President Reagan to initiate unfair-trading-practice complaints against West Germany, the European Community, South Korea, Brazil and Japan, and to take other actions against Canadian and Mexican trade.

Sen. Lloyd Bentsen (D-Tex.), chairman of the Senate Democratic Working Group on Trade Policy, and 12 other members of the group said in a letter to the president that some countries are engaging in unfair trading practices in the areas of telecommunications, aircraft and computers.

The request reflects the growing concern about the mounting trade deficit, which is likely to be a campaign theme for Democratic office-seekers this election year.

The administration is considering filing more complaints against countries to counter the congressional trade effort and is expected to discuss possible new cases at a meeting of the cabinet-level Economic Policy Council next week, sources said.

In the letter, the senators said:

*The Bundespost, the postal agency of the German government, annually purchases $5 billion in telecommunications equipment but that only $11 million in such equipment is purchased from U.S. firms. The Bundespost "simply refuses to buy such equipment from other than German producers," the letter said.

*The EC's sugar subsidies have "broken the back of the world sugar market" and have taken from the Caribbean Basin countries more than $3 billion in annual revenue.

*Japan's telecommunications markets still are closed to many U.S. goods such as fiber optics despite a long history of talks to get the Japanese to buy more U.S. goods.

*Brazil prevents the entry of foreign business aircraft into its markets. Meanwhile, Brazil's exports to the United States have increased.

*In South Korea, a U.S. company must apply to a government agency and the domestic industry for approval of a computer import plan, which in effect involves "commitments by large U.S. computer producers to locate facilities in Korea as a condition for being able to export their personal computers to that country."

A spokesman for the U.S. Trade Representative's Office said the department will take the letter seriously.

The senators have asked the president to act under Section 301 of the Trade Act of 1974, which gives the president broad powers to retaliate against countries he believes violate international trade laws or restrict U.S. commerce in an "unjustifiable, unreasonable or discriminatory fashion."

Reagan announced in September that for the first time he was initiating action under section 301 against Brazil, Japan and Korea and that the European Community and Japan satisfactorily settled pending 301 cases.

The senators also asked the president to make a high priority the opening of the Canadian telecommunications market and to urge the Mexican government to protect the patents of U.S. firms selling goods there as a prerequisite for admitting Mexico into the General Agreement on Tariffs and Trade.