Speaking to the German-British chamber of commerce in London, he said it was puzzling that Germany was second to bottom in putting single market directives into national law.
He urged the Germans to speed up compliance with directives and to play a full part in making the single market effective.
He had heard that the German federal system gave rise to difficulty, "but it is up to capitals to take responsibility for implementing treaty obligations."
Mr Lang also attacked Germany's decision in the council of ministers to vote against EU rules requiring notification when imports of goods from other member states were refused. The rules were eventually agreed despite German opposition. He had been saddened by Germany's decision.
"I know the German government was worried that the system would be difficult to operate, particularly in Germany's federal structure. Nevertheless we want to ensure that it does work well. It is for business's benefit."
If members observed the treaty and the case law of the European Court of Justice there should be few notifications, he added.
Mr Lang also criticised Germany for its failure to observe EU policy of mutual recognition of technical standards, which has led to protests from the Department of Trade and Industry in a number of cases. The DTI said examples were UK exports of gas analysing and measuring equipment and of tyre pressure gauges, where the Germans had insisted on compliance with their own domestic standards.
"It is trade and shared trading conditions that cement the partnership between the member states. But there are also areas where I would hope for improved co-operation."
No member state had a perfect record, but Denmark has put 99 per cent of single market directives into its own law.
"Making the single market work as effectively as possible has to be a joint as well as an individual effort; just as bad regulation has to be fought domestically as well as in the European Union, " he said.