E.g. the dropdown at https://meta.wikimedia.org/wiki/Special:Contributions?uselang=de shows German translations for the namespaces. This doesn't appear to be accessible through the Siteinfo API.
I was thinking uselang=de should provide the localized names https://meta.wikimedia.org/w/api.php?action=query&meta=siteinfo&siprop=namespaces&uselang=de but I am told it probably makes more sense to introduce a dedicated parameter, maybe sinslang (siteinfo namespace lang), or something similar.
This may be as simple as using the requested language instead of the content language in ApiQuerySiteInfo: https://gerrit.wikimedia.org/g/mediawiki/core/+/7462d3075a23095fd627cebeb0f66e081ad2bede/includes/api/ApiQuerySiteinfo.php#287