SELECT 
  a.*, 
  b.description 
FROM 
  cscart_currencies as a 
  LEFT JOIN cscart_currency_descriptions as b ON a.currency_id = b.currency_id 
  AND lang_code = 'ru'

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "a",
      "access_type": "ALL",
      "rows": 27,
      "filtered": 100
    },
    "table": {
      "table_name": "b",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["currency_id", "lang_code"],
      "ref": ["devmotorsbay.a.currency_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(b.lang_code = 'ru')"
    }
  }
}