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 = 'ru' 
  AND lang.name IN (
    'add_to_cart', 'sign_in_to_buy', 
    'text_login_to_add_to_cart', 'add_to_cart', 
    'sign_in_to_buy', 'text_login_to_add_to_cart'
  )

Query time 0.00103

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00943456,
    "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": 3,
          "cost": 0.00593204,
          "filtered": 100,
          "attached_condition": "lang.lang_code = 'ru' and lang.`name` in ('add_to_cart','sign_in_to_buy','text_login_to_add_to_cart','add_to_cart','sign_in_to_buy','text_login_to_add_to_cart')"
        }
      },
      {
        "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": ["lvshop.lang.lang_code", "lvshop.lang.name", "const"],
          "loops": 3,
          "rows": 1,
          "cost": 0.00350252,
          "filtered": 100
        }
      }
    ]
  }
}