API help is now localized. api.php's help output doesn't indicate this. You have to know to request it in another language with ?uselang=fr query string, which is mentioned in the Main module page.

There is some value in providing links to available localizations, especially if api.php could infer which languages had translations of the API module's messages. It doesn't make sense to rebuild the Universal Language Selector, but perhaps a lightweight chooser.

Note that if a logged-in user identifies her language preference, api.php output (and transclusions of it into wiki page via Special:ApiHelp) is localized, The discussion in T89768: Language support in Special:ApiHelp and its transclusion. suggests this isn't common, and has suggestions that apply to this bug. T89768 is about some wiki page explicity requesting a language for API help; this bug is about presenting a language choice to the user.