If you want help, you need to give us the complete query you have sent and the complete result you have got.

The full request URL I have been refering to is this:

https://api.themoviedb.org/3/search/movie?query=de&include_adult=false&language=en-US&page=1

where query is "de".

The resulting response looks like this:

{
  "page": 1,
  "results": [
     {
      "adult": false,
      "backdrop_path": "/tLAzri58mu0nFurXI10M6eYm0LD.jpg",
      "genre_ids": [
        18,
        10749
      ],
      "id": 173129,
      "original_language": "zh",
      "original_title": "金瓶風月",
      "overview": "A corrupt local official, Xi Men Qing, who lusts after women and money, pursues his brother's young wife, Lee Ping Er. Ping finds Men attractive, and under her drunken husband's nose, she and Men work their way through the illustrations of a pillow book, the \"Golden Lotus.\" Men pursues the same feat with his fourth concubine, Pan Jing Lien, who lives in the compound next to Ping. Jing's jealousy compounds a tale of humiliation, childbirth, death, and depravity. Does karma or enlightenment await anyone?",
      "popularity": 3.0094,
      "poster_path": "/nA1WbYl6Sllm71g8koNXhjWbMOc.jpg",
      "release_date": "1991-05-09",
      "title": "The Golden Lotus: Love and Desire",
      "video": false,
      "vote_average": 6.656,
      "vote_count": 48
    },

    {
      "adult": false,
      "backdrop_path": "/ygBB7R0TPBvFWwTzBk4rxp8eQcT.jpg",
      "genre_ids": [
        27,
        35
      ],
      "id": 246196,
      "original_language": "ja",
      "original_title": "レイプゾンビ3 LUST OF THE DEAD",
      "overview": "The zombies that arose after the nuclear outbreak are still high in numbers. The group of girls have lost some of their comrades and are fighting off the horny zombies to stay alive themselves. Can they stop the plague for once and for all?",
      "popularity": 4.5108,
      "poster_path": "/oeo54WatPOznX3cBJbJgDvevDJN.jpg",
      "release_date": "2013-06-23",
      "title": "Rape Zombie: Lust of the Dead 3",
      "video": false,
      "vote_average": 6.27,
      "vote_count": 98
    },
    {
      "adult": false,
      "backdrop_path": "/54mPXhtaUejdQtaCYeJPJP8ZfXX.jpg",
      "genre_ids": [
        18,
        10749
      ],
      "id": 715287,
      "original_language": "ko",
      "original_title": "새엄마의 욕망",
      "overview": "Sang-jin, who was envious of her neighbor after getting a beautiful wife, Ask your wife's friend Gian to take over the son's extracurricular teacher, Sang-jin, who left work early, accidentally sees Gian's shower. A man who wants to have sex with his son's tutor, And a son who wants to have a good time with a young stepmom, Ji-an and Jin-hee, who want money, are disappointed with her husband, and Bin-jin gives her body to a young son.",
      "popularity": 63.8128,
      "poster_path": "/rYC6UyML4CU4zYiZVbDMrwnGyWW.jpg",
      "release_date": "2020-05-29",
      "title": "Stepmom's Desire",
      "video": false,
      "vote_average": 7.36,
      "vote_count": 25
    },


  ],
  "total_pages": 500,
  "total_results": 10000
}

That is just a few of the movies that were in the response (I chose only those that include adult content).

For the full list of movies please go to https://developer.themoviedb.org/reference/search-movie and enter "de" in the query param.

If you need further info, don't hesitate to ask me. ;)

These movies are "adult:false", not "adult:true".

You will find here the definition of the adult flag.

Even though the movies are flagged as adult=false, can I somehow filter this kind of movies out?

Sorry, no.