SELECT 
  category_id, 
  category 
FROM 
  cscart_category_descriptions 
WHERE 
  category_id IN (921, 964) 
  AND lang_code = 'en'

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.41"
    },
    "table": {
      "table_name": "cscart_category_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "category_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 2,
      "filtered": "100.00",
      "index_condition": "((`s2cart`.`cscart_category_descriptions`.`category_id` in (921,964)) and (`s2cart`.`cscart_category_descriptions`.`lang_code` = 'en'))",
      "cost_info": {
        "read_cost": "1.21",
        "eval_cost": "0.20",
        "prefix_cost": "1.41",
        "data_read_per_join": "12K"
      },
      "used_columns": [
        "category_id",
        "lang_code",
        "category"
      ]
    }
  }
}