Charlotte's official website has been gradually unveiling details regarding its Blu-Ray and DVD releases, and has just recently confirmed that an unaired OVA episode will be released together with episode 13 in Volume 7 on the 23rd of March, 2016. That's still quite a while away, but in the
pretty much all english sites (or at least the once I know) that are going to sell them list it as not having subtitles and I dont remember those listings ever been wrong. Which just means, there still is a really little chance for it to have subs.
I remember when the little busters! Season 1 BDs were announced all English websites boasted that it came with English subtitles but when refrain was announced they didn’t say anything about it and it ended up not having any. If it is going you have subtitles I think we would have heard about it by now.