First, the good news: Clocks in Japan tick just the same as they do everywhere else in the world, so there’s no need to know the Japanese language to know the time.
That being said, it’s certainly helpful to know how to tell the time in Japanese, and understand all the nuances compared to English.
The time is given in 時 (ji, hours), 分 (fun, minutes) and, depending on the degree of precision, 秒 (byō, seconds). The number that contains the hour normally gets the Sino-Japanese reading, aka the 音読み (onyomi), except for 4 o’clock, which is 4yoji rather than shiji. For 7 o’clock, shichiji is more common than nanaji — though the latter is also possible. And 9 o’clock reads kuji, not kyūji.
