SELECT 
  lang.name, 
  IF(
    cscart_ult_language_values.value IS NULL, 
    lang.value, cscart_ult_language_values.value
  ) as value 
FROM 
  cscart_language_values lang 
  LEFT JOIN cscart_ult_language_values ON cscart_ult_language_values.name = lang.name 
  AND company_id = 2 
  AND cscart_ult_language_values.lang_code = lang.lang_code 
WHERE 
  lang.lang_code = 'ja' 
  AND lang.name IN (
    'returning_customer', 'password', 
    'forgot_password_question', 'error_incorrect_login', 
    'register_new_account', 'remember_me', 
    'returning_customer', 'password', 
    'forgot_password_question', 'error_incorrect_login', 
    'register_new_account', 'remember_me'
  )

Query time 0.00050

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "lang",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "392",
      "used_key_parts": ["lang_code", "name"],
      "rows": 6,
      "filtered": 100,
      "index_condition": "lang.lang_code = 'ja' and lang.`name` in ('returning_customer','password','forgot_password_question','error_incorrect_login','register_new_account','remember_me','returning_customer','password','forgot_password_question','error_incorrect_login','register_new_account','remember_me')"
    },
    "table": {
      "table_name": "cscart_ult_language_values",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "396",
      "used_key_parts": ["lang_code", "name", "company_id"],
      "ref": [
        "cscart_purchase_fl_com.lang.lang_code",
        "cscart_purchase_fl_com.lang.name",
        "const"
      ],
      "rows": 1,
      "filtered": 100
    }
  }
}