How to add Dutch in the language settings?

Is this for the Apple TV box? If so, which model?

Identify your Apple TV model - Apple Support

Find your Apple TV model number - Apple Support

1. You can set the device language (interface) to many languages, including Dutch. Settings﹥General﹥Apple TV Languages﹥…
2. You can set a preferred audio language to many languages, including Dutch. However, this depends on inclusion in the stream source, which many international productions may not have. (Dubbing in Dutch is more common in shows for young children.) Settings﹥General﹥Video and Audio﹥Audio Language﹥…
3. You can set a preferred subtitle, for when the audio language is different from that, and when offered by the stream provider. Settings﹥General﹥Video and Audio﹥Subtitle Language﹥…
4. During playback, you can see which audio languages and subtitle languages are available in the stream, and switch to any one of them. Change subtitles or languages for videos in the Apple TV app or iTunes Store - Apple Support

Apple TV box user guide (tvOS) - Apple Support

Is this for the Apple TV app on Mac with recent macOS version?

1. The Apple TV app interface language will by default follow the system language. Recent versions macOS also allow a language preference per app, to deviate from the system language. Change the language your Mac uses - Apple Support
2. The content audio language will follow the app language, when available, or use a default language (as set by the content provider).
3. The content subtitles in the TV app will remember the last used subtitle language, and try to use that again for a next video, until changed again.
4. During playback, you can see which audio languages and subtitle languages are available in the stream, and switch to any one of them. Change subtitles or languages for videos in the Apple TV app or iTunes Store - Apple Support

Apple TV app user guide for Mac - Apple Support