SELECT 
  cscart_state_descriptions.state 
FROM 
  cscart_states 
  LEFT JOIN cscart_state_descriptions ON cscart_state_descriptions.state_id = cscart_states.state_id 
  AND cscart_state_descriptions.lang_code = 'ja' 
WHERE 
  cscart_states.country_code = 'JP' 
  AND cscart_states.code = 'Tokyo'

Query time 0.00092

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "cscart_states",
      "access_type": "const",
      "possible_keys": ["cs", "code", "country_code"],
      "key": "cs",
      "key_length": "106",
      "used_key_parts": ["country_code", "code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_state_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["state_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}