SELECT 
  cscart_images.image_id, 
  cscart_images.image_path, 
  cscart_common_descriptions.description as alt, 
  cscart_images.image_x, 
  cscart_images.image_y 
FROM 
  cscart_images 
  LEFT JOIN cscart_common_descriptions ON cscart_common_descriptions.object_id = cscart_images.image_id 
  AND cscart_common_descriptions.object_holder = 'images' 
  AND cscart_common_descriptions.lang_code = 'en' 
WHERE 
  cscart_images.image_id = 84996

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "cscart_images",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["image_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_common_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "107",
      "used_key_parts": ["object_id", "lang_code", "object_holder"],
      "ref": ["const", "const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result