In reply to Marko Heijnen.

@Marko Heijnen

The post language selector only makes sense when you have a multilingual site and that percentage is not that high.

What @Manuel says, and: define “multilingual site”. ;) We usually think of translated content when we hear “multilingual”, don’t we? But that’s not the only use case for language-per-post. Matter of fact, that’s not the way we communicate as (mostly) bi- or multilingual human individuals. (I recommend Stepanie Booth’s talk from WordCamp Switzerland this year for excellent explanation of why confounding multi-/bilingual and translated content is such an unfortunate misconception.)

To put it clearly: I cannot see any justification for a “multilingual” WordPress core feature that would cover or even touch the broad issue of translation of authored content. Language relationships (and thus translation) imo can and should remain plugin territory.

Despite the title of this post showing the term “multilingual”, my proposal is not about bringing what we generally refer to as “multilingual functionality” to WordPress. The future feature or API we’re talking about imo should not cover relationships between languages.
There are several ways of implementing language relationships for specific use cases none of which should be preferred or excluded from the pallet of possible solutions by default.