Last modified: 2012-04-19 21:22:13 UTC
It appears that the reason list for Special:Block is in the language of the user who is currently viewing the page, instead of the content language (as when deleting or protecting a page, but that appears to be in the content language as it should be). It is a recent bug because this is not the case on WMF wikis.
cc'ing happy melon, since its probably related to the recent special:block re-write.
Fixed in r90895.