I think Youtube is a good example of how automatic translation can be less than helpful (I ended up installing a browser extension to force the original titles). And as you mention, it would really be nice to have not a single, but multiple languages which are not translated.
Bonus points if I can choose to which language other languages are auto-translated (I find auto-translations to English to be better on average than auto-translations to German).
What probably complicates things for the Discourse team: this would be a per-user thing, and not globally applicable to all users. E.g. thinking about a community in which I’m active: almost everyone speaks German, many users are also comfortable with English, and there’s a number of users who also read French (and then a handful of users with knowledge in other languages).
On top of that, there’s also question of mixed content. E.g. I will sometimes write German posts, but quote English articles or press releases directly. And if auto-translation is applied here, it would be nice to be able to show the original language just for the quoted part.
Similarly, if people are posting in multiple languages, and I’m proficient in one of them (e.g. post contains a French and an English part), I don’t want auto-translations by default. At least not for the whole post.
That being said, I do think the translation feature has value, and can provide people not proficient in a specific language with at least a basic understanding of what’s written there.