(Stepping back…)
There are two different language settings: (1) your current session’s locale, and (2) your default locale, which is applied when you log in.
I don’t think it’s possible to change the current session locale via REST (though it might just work if you append ?lang=xyz to any request, because of the LocaleChangeInterceptor in the legacyui module, but I don’t know if this catches REST requests). We should add an intentional feature for this, e.g. something like a user/uuid/locale subresource with explicit properties for currentSessionLocale and defaultLocale.
Also, we should improve the REST API so that the user properties are exposed individually as some sort of intentional resource, rather than as an obscure json object.