Honestly rather than mucking about with language settings, I'd just brute-force it with a lookup function:
=MATCH(LOWER(A1), {"janeiro","fevereiro","março","abril","maio","junho","julho","agosto","setembro","outubro","novembro","dezembro"}, 0)
Or maybe just match the first three letters:
=MATCH(LOWER(LEFT(A1, 3)), {"jan","fev","mar","abr","mai","jun","jul","ago","set","out","nov","dez"}, 0)
It's pretty brute force, but I feel like it's a more transparent approach that will be more independent of language settings, and also more apparent to debug if something goes wrong in the future.
You could even get really slick and nest with IFERROR, or build a SUMPRODUCT, that will assess for both English and Portuguese.
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