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 = 1 
  AND cscart_ult_language_values.lang_code = lang.lang_code 
WHERE 
  lang.lang_code = 'en' 
  AND lang.name IN (
    'returning_customer', 'email', 'password', 
    'forgot_password_question', 'register_new_account', 
    'remember_me', 'returning_customer', 
    'email', 'password', 'register_new_account', 
    'remember_me', 'forgot_password_question', 
    'email', 'password', 'register_new_account', 
    'returning_customer', 'email', 'password', 
    'forgot_password_question', 'register_new_account', 
    'remember_me', 'returning_customer', 
    'email', 'password', 'register_new_account', 
    'remember_me', 'forgot_password_question', 
    'email', 'password', 'register_new_account'
  )

Query time 0.00048

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_ult_language_values",
      "access_type": "system",
      "possible_keys": ["PRIMARY"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "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,
      "attached_condition": "lang.lang_code = 'en' and lang.`name` in ('returning_customer','email','password','forgot_password_question','register_new_account','remember_me','returning_customer','email','password','register_new_account','remember_me','forgot_password_question','email','password','register_new_account','returning_customer','email','password','forgot_password_question','register_new_account','remember_me','returning_customer','email','password','register_new_account','remember_me','forgot_password_question','email','password','register_new_account')"
    }
  }
}

Result