SELECT 
  category_id, 
  category 
FROM 
  cscart_category_descriptions 
WHERE 
  category_id IN (6, 12) 
  AND lang_code = 'ja'

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_category_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["category_id", "lang_code"],
      "rows": 2,
      "filtered": 100,
      "index_condition": "cscart_category_descriptions.category_id in (6,12) and cscart_category_descriptions.lang_code = 'ja'"
    }
  }
}