"市镇自治机关" can be translated as "municipal autonomous organs" or "town/township self-governing bodies", depending on the specific context and the level of government being referred to.
"Municipal autonomous organs" is more formal and might be used in official documents or legal contexts, where "municipal" refers to a city or town with its own local government, and "autonomous organs" implies self-governing bodies with certain powers.
"Town/township self-governing bodies" is more straightforward and might be used in everyday conversation or less formal settings. Here, "town" or "township" specifies the type of local government, and "self-governing bodies" clearly indicates that these are organizations with self-governing authority.
In many cases, the choice between these translations may depend on the specific legal or administrative framework in the country or region being discussed.