language fallback: it_CH (Italian, Swiss) -> it_IT (Italian, Italy)

Add Silesian and Kashubian to language fallback map Silesian and Kashubian speakers would prefer to fall back to Polish (pl) translations instead of English (C). https://bugs.freedesktop.org/show_bug.cgi?id=89021

Add language fallback map This map will be used to provide a fallback for translations. For example, a Niederdeutsch (nds) speaker prefers to fall back to German (de) translations rather then the English (C) ones. https://bugzilla.redhat.com/show_bug.cgi?id=624158#c9