After update of the GP Premium plugin and Polylang plugin, string translations in Polylang plugin became disabled. And all translations returned to the defaults. It’s happened not only for GP, but for the WordPress core and Rank Math plugin. At the same time, Cookie Notice plugin translations wasn’t hurt.
Seems like in general, despite disabled checkboxes, in case of changes “Save” button has effect. Still not clear why checkboxes are disabled. But I think this topic can be closed. Really doesn’t look like an issue with GP.
Thanks, anyway, Leo