When watching films in languages I can understand but which are foreign to me, I try to follow the spoken dialogue and read the subtitles at the same time. This is quite a challenge. When the subtitles are not there, I can usually follow the dialogue better because my concentration isn’t split between listening to one language and reading another.
What is even more challenging is watching a foreign film with subtitles in a language other than English. For example, while in Taiwan I saw a few films in French and Taiwanese with Chinese subtitles. Trying to understand them was hard work.
Foreign films or TV programmes with subtitles in the same language are actually easier to follow than those with no subtitles. In China and Taiwan for example, most films have subtitles in Chinese because not everybody there can understand Mandarin.