Hello everyone
when I add a link from YouTube, it is converted, but if a user has a different language, the link is only displayed without the video embedded.
Since I am writing the post in German, it may also be reproducible here:
https://www.youtube.com/watch?v=NU9JoFKlaZ0
Here is a picture of the original language
English language:
That is a know issue with translated posts, we only do oneboxes for the original language of the post.
We are working on a fix.
Small update:
I just deactivated the plugin GitHub - discourse/discourse-lazy-videos: Lazy loading for embedded videos to try it out. Now the YouTube videos are also displayed in other languages.
Furthermore, it also works if I add a German trailer in the German translation and an English trailer in the English translation. I am satisfied for now.
Maybe this will help someone 
We recently updated translations to have proper lightboxing and oneboxing - FEATURE: Add oneboxes and lightboxes to localized posts by xfalcox · Pull Request #34900 · discourse/discourse · GitHub.
We should be able to make video oneboxing work as well soon™.