This looks suspiciously similar to what LTeX produces for me. Are you sure that this is not the true origin of the error? If this is indeed LTeX, you will see it in :LspInfo
.
If so, here is some info about changing the language of LTeX: https://valentjn.github.io/ltex/advanced-usage.html In short, you could try \usepackage[french]{babel}
, or % LTeX: language=fr-FR
.