Default language is English; detect the browser language (Polish -> pl, Ukrainian -> uk, otherwise en) and persist the user choice. Add full Ukrainian (Cyrillic) translations, a language switcher showing native endonyms, and keep vue-i18n, Vuetify locale, <html lang> and document.title in sync.
Add vue-router routes with accessible navigation (document title + focus management), Pinia session store, vue-i18n (PL/EN) and the default layout. Replace the landing page with routed views and remove the old MainView.
Rafał Tyński