Polylang intentionally keeps the title attribute empty for the language switcher. As this field is empty some themes copy the navigation label in this attribute. It’s a bad practice as it doesn’t add any useful information and the title can contains only text whereas the navigation label can include images.
Of course, the default WordPress Twenty themes haven’t such issue. Please ask to your theme support how to solve this. A quick fix is to remove the flags in the language switcher options.