i18n: Global language setting for bones

I got this idea during last viur-meeting, when we talked about language simple: Why does every bone provides its own list of languages, when it's multi-lang? In general, a project keeping data in multiple language should handle exactly the same languages everywhere. Therefore, why isn't there a global option for this?

Currently, this can be solved by a project-specific setting or by use of conf.i18n.languages.