use Illuminate\Validation\Rule;
 public function rules()
    {
        $rules = [          
            'name_eng'=>'required|string',
            'field1' => [
                'required',
                Rule::unique('table_name')->where(function ($query) {
                    $query->where('field2', $this->get('field2'));
                })
            ],  
        ];
        return $rules;
    }