Pauline, I may not be understanding exactly what you're saying, but the crux on here is that you can follow hashtags. Just click on one and the follow button appears at the top.
Equally, this reminds me of your mention of wanting to see posts in other languages. Likewise, just follow a hashtag in that language, e.g. 'climat'.
Also, you can do as I did when I joined and either pin a post with hashtags, or put them in your bio. Hashtags are the thing.
@gsymon
Yes, you can follow hashtags, but that doesn't guarantee that you'll see everything on the hashtag. You'll see all the posts with that hashtag _which have reached your instance_.
There's another kind of thing, I think it's called a group, where you include an @ for a special kind of account and it forwards everything sent to it. With those, because of the proactive forwarding, you _can_ see everything sent using the @.
@pvonhellermannn @inquiline