"区镇的地方长官" can be translated into English as "the local magistrate or official in charge of a district or town".
Here, "magistrate" often refers to a local government official with administrative or judicial powers in some regions, while "official in charge" more directly conveys the idea of someone responsible for managing or overseeing a specific area. Depending on the specific context and the level of authority the official holds, you might also use terms like "district administrator," "town mayor" (if referring specifically to the mayor of a town), or "local government leader" for a more general description.
For a more precise and context-appropriate translation, you might need to consider the specific role and responsibilities of the "地方长官" in question.