SELECT 
  cscart_ab__is_attributes.*, 
  is_descriptions.*, 
  TRIM(
    CONCAT(
      is_descriptions.prefix, is_descriptions.text, 
      is_descriptions.suffix
    )
  ) AS value 
FROM 
  cscart_ab__is_attributes 
  LEFT JOIN cscart_ab__is_attribute_descriptions as is_descriptions ON cscart_ab__is_attributes.is_id = is_descriptions.is_id 
  AND lang_code = 'en' 
WHERE 
  `object_type` IN ('C', 'P', 'F') 
  AND `storefront_id` = 1

Query time 0.00245

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.69"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_ab__is_attributes",
          "access_type": "ref",
          "possible_keys": [
            "storefront_id"
          ],
          "key": "storefront_id",
          "used_key_parts": [
            "storefront_id"
          ],
          "key_length": "4",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 7,
          "rows_produced_per_join": 2,
          "filtered": "30.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.21",
            "prefix_cost": "0.95",
            "data_read_per_join": "33"
          },
          "used_columns": [
            "is_id",
            "storefront_id",
            "attribute_type",
            "object_type"
          ],
          "attached_condition": "(`s2cart`.`cscart_ab__is_attributes`.`object_type` in ('C','P','F'))"
        }
      },
      {
        "table": {
          "table_name": "is_descriptions",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "is_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "s2cart.cscart_ab__is_attributes.is_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.53",
            "eval_cost": "0.21",
            "prefix_cost": "1.69",
            "data_read_per_join": "4K"
          },
          "used_columns": [
            "is_id",
            "lang_code",
            "prefix",
            "text",
            "suffix"
          ]
        }
      }
    ]
  }
}