Use in child theme's functions.php. Useful to translate a handful of strings. For more extensive translations a plugin like Loco Translate comes in handy.

WordPress simple text translations

// Theme translations
  function custom_text_translate($translated) {
	  switch ( $translated ) {
		  case 'The Shop' :
			$translated = 'Donate';
		  break;
		  case 'Welcome to our online store. Take some time to browse through our items.' :
			$translated = 'Welcome to our donations hub.';
		  break;
	  }
	return $translated;
  }
  add_filter('gettext', 'custom_text_translate' );