INT file is just used for localization. Basically you can flag string data inside a UE game to load its string out of the INT file, and its much easier to package localized text into a set of files that the engine knows to use instead of the standard english text files. Unreal's pretty flexible in regards to how they work, and in theory you could change non-text data through the INT file in some cases. Also depends how a company sets up their UE game to use the data in some cases. However, its commonly the case that you might add in text data to the INT file in preliminary development in order to proxy out some part of the game, but when it comes to the final recording of audio files the text ultimately changes. You don't want your preliminary test levels to break and the extra text doesn't do any damage or cause issues -- so no reason to remove it. I doubt that there is corresponding audio for the mission briefings in question.