Starting with Mastodon 3.5, we no longer try to auto-detect your posting language. It defaults to your interface language, unless overriden by setting it explicitly per-post in the API, or setting in Preferences -> Other.
And if you want to pop up in the Explore tab, make sure to opt-in to the Suggest account to others preference on the Edit profile page and set your posting language correctly.
@Gargron thank you