"用旗语发信号" can be translated into English as "to signal using flag signals" or more naturally as "to send signals by semaphore".
"Semaphore" specifically refers to the system of signaling, typically with flags or lights, used especially at sea or in military contexts.
"To signal using flag signals" is a more literal and straightforward translation, while "to send signals by semaphore" is a more idiomatic and commonly used expression in English.