The Language filter (Creator Filter section) restricts results to creators whose detected main content language matches your selection, e.g. German or English. The language is derived from the creator's content, not from the country they live in — a creator in Germany who posts in English will be found under English.
Caveat: many creators post in several languages (especially English alongside their local language). A strict language filter can therefore exclude relevant creators. If you want to reach German-speaking audiences, filtering by country (creator location or audience country) and leaving the language filter empty usually yields better coverage.