Because that translates everything to everything. What I suggest is unsupported would be translated to supported ones in post level. Big difference.
We have a forum defaulted to German. I write there in Finnish [1] and it will be translated to Germany. English speakers can’t understand a bit, even it would be a supported language and de facto standard globally.
Sure, what happen when someone replies in Germany, because that user saw translated content in Germany. We have a mess.
I don’t know how to solve this issue. But this is a questionmark in non-English forums and the demand start to be that everyone outside massive languages must be at least bi-lingual. Well, we quite often are 
Anyway. It is nice add-on that post can be written in any language. My issue is that is such situation global audience can’t read it, unless they can default language, because understanding supported ones is not enough.
Non of chinese can’t undestand my previous post written in Finnish, unless they can English too.
such behaviour is really problematic, and in the past at the age before social media we had big issues with one nationality; but then was then and now is now ↩︎