SELECT 
  tax 
FROM 
  cscart_tax_descriptions 
WHERE 
  tax_id = 6 
  AND lang_code = 'ar'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.00"
    },
    "table": {
      "table_name": "cscart_tax_descriptions",
      "access_type": "const",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "tax_id",
        "lang_code"
      ],
      "key_length": "9",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.00",
        "eval_cost": "0.10",
        "prefix_cost": "0.00",
        "data_read_per_join": "784"
      },
      "used_columns": [
        "tax_id",
        "lang_code",
        "tax"
      ]
    }
  }
}