We suggest you to try using this plugin: Loco Translate. It can be used for changing default theme labels and messages either.
Install the plugin and go to Loco Translate -> Themes to add a new language or edit an existing one:
If the translation doesn’t work or some words cannot be found, make sure that the site has only one copy of the theme installed and then press the ‘Sync’ button in the Loco Translation page. You can find more info in this plugin documentation.
Please consider, this plugin is meant to be used for theme interface translation. If you want to translate website content (create a multilanguage website), please use WPML plugin.