translate the page title in search template file
Drupal 8 has extensive multi-lingual support and French is one of the supported languages.

As administrator, turn on the language module at Administration -> Extend

Then go to Administration -> Configuration -> Regional and Language -> Languages, click on Add Language and select French. If you want it to be the default language set it as the default and Save Configuration. (This can cause issues if the site is already live!)

The User Interface Translation module (also in core) can be used to import additional translations and to allow you to translate strings manually as needed.