WPML

Do you have try WPML String Translation Add-on: https://wpml.org/documentation/getting-started-guide/string-translation/#download-and-activate-string-translation

Or there is a way to register strings for translation via code: https://wpml.org/documentation/support/translation-for-texts-by-other-plugins-and-themes/

We use Polylang ( https://de.wordpress.org/plugins/polylang/ ) for our projects.