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.