Locale missing in nextcloud.fpm docker container

Hi,
I am trying to get Nextcloud up and running using the docker fpm image with nginx web- and proxyserver and letsencrypt for the SSL certs.

Currently I am struggling wiht some error messages on the website:

Das Setzen der Spracheeinstellung auf en_US.UTF-8/fr_FR.UTF-8/es_ES.UTF-8/de_DE.UTF-8/ru_RU.UTF-8/pt_BR.UTF-8/it_IT.UTF-8/ja_JP.UTF-8/zh_CN.UTF-8 ist fehlgeschlagen.

Bitte installiere eine dieser Sprachen auf Deinem System und starte den Webserver neu.

I am using the following docker-compose.yaml to start the container:

version: '3.1'

services:
  proxy:
    image: jwilder/nginx-proxy
    ports:
      - 9095:80
      - 9445:443
    volumes:
      - ./proxy/conf.d:/etc/nginx/conf.d
      - ./proxy/vhost.d:/etc/nginx/vhost.d
      - ./proxy/html:/usr/share/nginx/html
      - ./proxy/certs:/etc/nginx/certs:ro
      - /var/run/docker.sock:/tmp/docker.sock:ro
    networks:
      - proxy-tier
    restart: always

  letsencrypt-companion:
    image: jrcs/letsencrypt-nginx-proxy-companion
    volumes:
      - ./proxy/conf.d:/etc/nginx/conf.d
      - ./proxy/vhost.d:/etc/nginx/vhost.d
      - ./proxy/html:/usr/share/nginx/html
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - ./proxy/certs:/etc/nginx/certs:rw
    restart: always

  web:
    image: nginx
    links:
      - app
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
      - ./www:/var/www/html:ro
      - ./nextcloud/apps:/var/www/html/apps
      - ./nextcloud/custom_apps:/var/www.html/custom_apps
      - ./nextcloud/config:/var/www/html/config
      - ./nextcloud/data:/var/www/html/data
    environment:
      - VIRTUAL_HOST=192.168.178.3,li2laden.selfhost.eu,192.168.100.10
      - VIRTUAL_NETWORK=nginx-proxy
      - VIRTUAL_PORT=80
      - LETSENCRYPT_HOST=li2laden.selfhost.eu
      - LETSENCRYPT_EMAIL=support@graf-online.me
    ports:
      - 9096:80
    networks:
      - proxy-tier
    restart: always

  app:
    image: nextcloud:fpm
    volumes:
      - ./www:/var/www/html
      - ./nextcloud/apps:/var/www/html/apps
      - ./nextcloud/custom_apps:/var/www/html/custom_apps
      - ./nextcloud/config:/var/www/html/config
      - ./nextcloud/data:/var/www/html/data
    networks:
      - proxy-tier
    restart: always
    env_file:
      - nextcloud.env
    secrets:
      - NEXTCLOUD_ADMIN_USER
      - NEXTCLOUD_ADMIN_PASSWORD
      - MYSQL_PASSWORD
      - MYSQL_DATABASE
      - MYSQL_USER
      - SMTP_PASSWORD
    
    
networks:
  proxy-tier:
    external:
      name: nginx-proxy

secrets:
  NEXTCLOUD_ADMIN_USER:
    file: ./NEXTCLOUD_ADMIN_USER.txt
  NEXTCLOUD_ADMIN_PASSWORD:
    file: ./NEXTCLOUD_ADMIN_PASSWORD.txt
  MYSQL_PASSWORD:
    file: ./MYSQL_PASSWORD.txt
  MYSQL_DATABASE:
    file: ./MYSQL_DATABASE.txt
  MYSQL_USER:
    file: ./MYSQL_USER.txt
  SMTP_PASSWORD:
    file: ./SMTP_PASSWORD.txt

As far as I understood - there is no way to install additional languages on the nextcloud.fpm image and setting the locale to LC_ALL=C.UTF-8 didn’t help.

Any ideas what am I doing wrong?

Thanks