One of the advantages of living in a democracy is that you're allowed to say whatever you're thinking.
Literal
Democracy-state [at-に] live advantage's one [topic-は], self [subject-が] is-thinking thing [object-を] whatever-it-is say thing [subject-が] is-permitted thing [copula-である].
Two stacked nominalizations worth following. 住む利点 is a relative clause modifying 利点: 'the advantage (of) living.' Then the sentence-end こと chains nominalize actions — 言うこと ('the act of saying') and 許されること ('being permitted') — so the final predicate is the whole abstract nominal 'the thing permitted is saying whatever one thinks.' である is the formal/written copula, here closing a definition-like statement.