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 = 'es' 
  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 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0174097,
    "nested_loop": [
      {
        "table": {
          "table_name": "lang",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "392",
          "used_key_parts": ["lang_code", "name"],
          "loops": 1,
          "rows": 6,
          "cost": 0.01122386,
          "filtered": 100,
          "attached_condition": "lang.lang_code = 'es' 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": [
            "u358358020_main.lang.lang_code",
            "u358358020_main.lang.name",
            "const"
          ],
          "loops": 6,
          "rows": 1,
          "cost": 0.00618584,
          "filtered": 100
        }
      }
    ]
  }
}