Binary package hint: ubiquity
This is a partial recurrence of bug 276657, but I think it's best to file it separately.
Before the 8.10 beta release, we discovered that file-copy blacklisting (from the dvd-performance-hacks specification) was broken, and disabled it. After the beta, we committed a change which at the time was believed to fix it. Unfortunately, it still got the blacklist somewhat wrong, having the following problems:
* The code to determine which language packs should remain installed did not run before generating the blacklist. As a result, the blacklist generation could not possibly take language packs into account.
* Packages with prerm scripts were filtered out before calculating which packages could be removed, effectively treating them as if they were sacrosanct packages from the standard desktop installation. This meant that any dependencies of a language pack (or one of its dependencies) with a prerm would never have their files blacklisted.
* ubiquity only tried one level of broken-package removal using python-apt. Due to the quirks of that interface you actually need to repeat removals until you stop making progress.
* Packages were removed from the difference list (the list being compared against to check whether packages broken by a removal were allowed) once they were themselves verified to be safely removable. The effect of this was that they stopped being considered as packages that were allowed to be removed while processing subsequent packages.
While fortunately beta installations were unaffected, I think this would be worth a release notes item for people who installed from post-beta (or indeed pre-beta, while we're at it) daily builds and may have some missing files. I suspect that deselecting the language in language-selector, committing, and then going back in and reselecting the language would sort it out, but I'll need to check whether deselecting a language goes further down the dependency graph than just the top-level language packs.