Your camera has tagged the video, in the meta-data, as being 23.976. Presumably this is the frame rate you had set for regular recording. Since it was recorded at 120, when you play it back, you get a 5x slow-mo.
This is usually what people want when they record at such a high frame rate, because there's generally not much point in recording at 120 otherwise. But if you want to record at 120 and play back at 120, then you will need to tell your video editing software. Create a timeline at 120 FPS, and when you import the video, tell the software that it's 120FPS, or just speed it up 5 times.
If your video editor timeline is less than 120FPS, like if it's 24 FPS, then there's absolutely no point in shooting at 120.
If you're not using editing software, then I think there's still no point in shooting at 120.
