It should be possible to vary skin and/or core styles based on language, preferably in a way that does not make dealing with large groups of languages onerous. See T73240 for motivation (in that case it is about content language; it's not hard to imagine other use cases which would need varying on user interface language, so ideally both should be supported).
Old patch from @Jdlrobson: https://gerrit.wikimedia.org/r/#/c/125760/