How to set required field only for default language?

I have this input, where i enter for languages some text. Now i want to set required field but only if its language code [de]. When i do like this i get required field for all languages. Any suggestion how can i fix this?

  @foreach ($languages as $language)
        <input type="text" id="text-title" name="article_title[]" value="" class="form_input"  />
    @endforeach

  public function rules()
    {
        return [
            'article_title[de]'             => 'required:articles',
            'slug'                          => 'required|unique:articles',
            'article_intro[de]'             => 'required:articles',
            'article_content[de]'           => 'required:articles',
            'article_category[de]'          => 'required|exists:categories,id',
        ];
    }

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