SELECT 
  a.code, 
  b.state 
FROM 
  cscart_states as a 
  LEFT JOIN cscart_state_descriptions as b ON b.state_id = a.state_id 
  AND b.lang_code = 'en' 
WHERE 
  a.country_code = 'GB' 
ORDER BY 
  b.state

Query time 0.00071

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "b.state",
      "temporary_table": {
        "table": {
          "table_name": "a",
          "access_type": "ALL",
          "possible_keys": ["cs", "country_code"],
          "rows": 666,
          "filtered": 20.72072029,
          "attached_condition": "a.country_code = 'GB'"
        },
        "table": {
          "table_name": "b",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["state_id", "lang_code"],
          "ref": ["fluentmart_cscart.a.state_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(b.lang_code = 'en')"
        }
      }
    }
  }
}