It depends. There is a toggle near the timeline which allows you to decide whether to see everything in the language it was written in or to translate everything into your interface language. That works for all languages you can select in the header.
So for example in this topic there is a mix of German and English. My interface language on Meta is English. I can read the posts in the language they were written in or read everything in English.
I prefer to post in English because automatic translations are not perfect, and I like to control the version most users will read (or can at least use as a fallback when the automatic translation seems strange). Things get complicated when setting names are translated or when translators use slightly different terms when they translate the Discourse interface. For example in a German post I’d use “Team” for staff because that’s the name of the group when your interface is German. But AI will translate “Team” without that knowledge and there is no default “team” group in English. So it could easily be mixed up with the team group here at Meta.