Sennheiser does not recommend using a product outside of it's intended country of sale/use because different versions of products exist to comply with specific rules and regulations of specific countries. These rules and regulations are determine by the individual countries. If a product that is sold in the United States is intended for use outside of the United States it is the responsibility of the user to check regulations for other countries to ensure that the product remains compliant with the law of the country of use. Sennheiser strongly recommends purchasing a product within the country that the product will be used in to ensure that the product complies with local regulations.
Please be aware that the RS 120 is a radio frequency model headphone and rules and regulations about which frequencies can be used change from country to country and it is the government of the respective country that creates these regulations. For this reason Sennheiser manufacturers different models using different frequency ranges for different parts of the world. The RS 120 models sold in the United States operate at 926-928 Mhz range and are designed to comply with the regulations determined by the United States Government.
Furthermore the power supplies and electronics for products are optimized for the country that the product will be used in. For example products sold in the United States have been optimized for use on the 110-120V 60 Hz circuits. With the use of an adapter the power supply should work outside of the intended country of use however if the power supply is used outside of the intended country of use it is not running optimally and the potential for the circuitry to wear out more quickly is much higher.