"남자들이 영어로 이야기합니다."
Translation:The men talk in English.
The difference is how the words get used in English. "To talk" and "to speak" are used slightly differently; "the men speak English" means the men are able to use English when talking to people, while "the men talk in English" means the men are, right now, actively using English to talk to each other. The phrase "in [language]" is used to emphasize that the language mentioned is being used for the action. Consider "this is written in Japanese", which is used to identify the language used to write as Japanese, versus "this is written Japanese", which would be used to indicate an excerpt as an example of how Japanese is written.