When using language and include_image_languages together, the language specified in the 'language' parameter is ignored e.g.

https://api.themoviedb.org/3/movie/343611/images?api_key=APIKEY&language=de&include_image_language=en,null

Does not return any German images even though they exist.

{
  "id": 343611,
  "backdrops": [
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/4ynQYtSEuU5hyipcGkfD6ncwtwz.jpg",
      "height": 2160,
      "iso_639_1": null,
      "vote_average": 5.384,
      "vote_count": 2,
      "width": 3840
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/yZ2YcgwI9DbCxdao22nIwz3L3sB.jpg",
      "height": 1080,
      "iso_639_1": null,
      "vote_average": 5.246,
      "vote_count": 2,
      "width": 1920
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/m5d6rpPcWooIwO7Znu0u9edUmJm.jpg",
      "height": 1080,
      "iso_639_1": null,
      "vote_average": 5.24542124542125,
      "vote_count": 2,
      "width": 1920
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/cSfoVPf09W5nDNm3BElFB9uIIjl.jpg",
      "height": 2160,
      "iso_639_1": null,
      "vote_average": 5.23809523809524,
      "vote_count": 2,
      "width": 3840
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/ziSRDLhYBmAaYuY5eSib0RVvUWE.jpg",
      "height": 1080,
      "iso_639_1": null,
      "vote_average": 0,
      "vote_count": 0,
      "width": 1920
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/gLXq0jbJQeyY0jw71F6buwcVYZl.jpg",
      "height": 720,
      "iso_639_1": null,
      "vote_average": 0,
      "vote_count": 0,
      "width": 1280
    },
    {
      "aspect_ratio": 1.77662337662338,
      "file_path": "/6U0vV2ddgWEtoFizAfLhDWZYJya.jpg",
      "height": 770,
      "iso_639_1": null,
      "vote_average": 0,
      "vote_count": 0,
      "width": 1368
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/60ihNKJfnJImKllEjktirVqRWwb.jpg",
      "height": 720,
      "iso_639_1": null,
      "vote_average": 0,
      "vote_count": 0,
      "width": 1280
    },
    {
      "aspect_ratio": 1.77935943060498,
      "file_path": "/aODL7dgalKi8JfHQUF7xaRNDisY.jpg",
      "height": 843,
      "iso_639_1": null,
      "vote_average": 0,
      "vote_count": 0,
      "width": 1500
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/3DZRVOO6jxLm0Duf0i1i0iFr8e1.jpg",
      "height": 1080,
      "iso_639_1": "en",
      "vote_average": 0,
      "vote_count": 0,
      "width": 1920
    },
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/9UsSZ9BOT2LsRR2bDOCE7Sg4qSp.jpg",
      "height": 1080,
      "iso_639_1": "en",
      "vote_average": 0,
      "vote_count": 0,
      "width": 1920
    }
  ],
  "posters": [
    {
      "aspect_ratio": 0.640625,
      "file_path": "/mbEtKblZkcwMabUJT8Xs0YjPMhU.jpg",
      "height": 2048,
      "iso_639_1": "en",
      "vote_average": 5.456,
      "vote_count": 5,
      "width": 1312
    },
    {
      "aspect_ratio": 0.666666666666667,
      "file_path": "/IfB9hy4JH1eH6HEfIgIGORXi5h.jpg",
      "height": 3000,
      "iso_639_1": "en",
      "vote_average": 5.3125,
      "vote_count": 1,
      "width": 2000
    },
    {
      "aspect_ratio": 0.666666666666667,
      "file_path": "/j76Ag7aa1dnhPCu5xxBERNKlL55.jpg",
      "height": 1500,
      "iso_639_1": "en",
      "vote_average": 5.172,
      "vote_count": 1,
      "width": 1000
    },
    {
      "aspect_ratio": 0.666666666666667,
      "file_path": "/zSCRnAwBxQDwIKLlCpPnw6hsEYl.jpg",
      "height": 1500,
      "iso_639_1": "en",
      "vote_average": 5.172,
      "vote_count": 1,
      "width": 1000
    }
  ]
}

If you specify the German filter without the fallback you get the results:

https://api.themoviedb.org/3/movie/343611/images?api_key=APIKEY&language=de

{
  "id": 343611,
  "backdrops": [
    {
      "aspect_ratio": 1.77777777777778,
      "file_path": "/uDIbB3IaekjTD5SyJyXCujR56sr.jpg",
      "height": 1080,
      "iso_639_1": "de",
      "vote_average": 0,
      "vote_count": 0,
      "width": 1920
    }
  ],
  "posters": [
    {
      "aspect_ratio": 0.706770833333333,
      "file_path": "/aqsrCfeU3gYSiJRzMR8IY2oFshy.jpg",
      "height": 1920,
      "iso_639_1": "de",
      "vote_average": 0,
      "vote_count": 0,
      "width": 1357
    }
  ]
}