In T309113: Static pages fail to build when the active locale is not also supported by mkdocs-material theme we found out that the l10n system that the mkdocs-material theme has invented for itself raises unhandled exceptions when the theme's language setting is set to a language which does not have an existing data file in the partials/languages/ directory. We have worked around this in a way that is avoiding fatal errors, but we will still be rendering pages in languages where we are forced to use the English default messages rather than localized versions.
One potential fix for this would be to add a second message catalog for the theme to our data at translatewiki.net so that we could crowdsource translations for the theme's strings from the same community that is providing us with translations for the content message catalog.
Inside the theme, a t( msg ) macro implements their translation system. This macro is defined in partials/language.html and could be replaced with any implementation we needed. Calls to the macro look like {{ lang.t('header.title') }}. banana-i18n would be a very reasonable technology to use in a replacement.