Widget titles are not listed in the strings translation table

If you enter a title in the widget options and this title is not listed in the translation table, it is most likely because the widget uses the very old Widget API (WP < 2.8). Obviously the advantage for the widget is to be compatible with WordPress older than 2.8 but unfortunately, there is no way to make Polylang automatically compatible with all widgets which use this old API.
It is however possible to make them compatible for people who have some PHP skills, using the Polylang API functions 『pll_register_string』 and 『pll__』.