Language recognition by homomorphisms is a central construction of algebraic language theory. Initially studied for monoids and semigroups, it has subsequently been expanded to other algebraic structures. Our new categorical account is based on fibrations, which have already seen other applications in automata theory. Languages and surjective homomorphisms give indeed rise to two fibrations, and the notion of language recognition is stable under reindexing. We develop this framework in a category with a factorisation system and address two main technical questions in the fibrational setting. First, we provide sufficient conditions under which languages have syntactic quotients (which is a generalisation of syntactic congruences) and we show how such quotients can be described in some concrete cases using a result by Slomiński. Second, we introduce sufficient conditions under which (regular) languages are closed under certain J-limits and J-colimits.

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