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

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_gr__is_attributes",
      "access_type": "ref",
      "possible_keys": ["storefront_id"],
      "key": "storefront_id",
      "key_length": "4",
      "used_key_parts": ["storefront_id"],
      "ref": ["const"],
      "rows": 7,
      "filtered": 100,
      "attached_condition": "cscart_gr__is_attributes.object_type in ('C','P','F')",
      "using_index": true
    },
    "table": {
      "table_name": "is_descriptions",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["is_id", "lang_code"],
      "ref": ["noframes_kz_new.cscart_gr__is_attributes.is_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(is_descriptions.lang_code = 'ru')"
    }
  }
}