The language is properly set. The post itself is in English. The language setting defines the language of the post, not the language of the linked content.
When you submit a post or comment, there is a selection box to indicate what language it is. People can choose what language they do understand to filter out content they can not / are not interested in seeing. OP has selected "English" as the language of a post that is clearly in German.
Because this isnt German local news or something, this is a presentation on the inner workings of the railway map. Might very well be interesting to people regardless of language, you could probably turn off the audio and watch it with subtitles.
On principle you are correct though, the language setting isnt properly used most of the time.
Might very well be interesting to people regardless of language
Then why bother with language settings at all? If people are just going to set the languages according to what they think might be relevant to the audiences, then the whole idea becomes a silly variant of https://www.youtube.com/watch?v=rZ3ETK7-ZM8