On French Language, we consider duplicates to be duplicates regardless of the language.
Given that answers are not always in the same language as the question, it wouldn't really make sense otherwise. If an answer in French to a question in English is legitimate, it's also legitimate for a duplicate question in English.
When we do this, we try to make sure to translate at least the top answers of the original question so that they're available in both languages. (Example: a question in English was closed as a duplicate of this one on 2016-07-24.)
(Feel free to edit this answer to add a Spanish translation.)