Instrumentation for gathering translation statistics

OEMs would love to have information on which languages are well-supported. As well as software-level support (information being collected elsewhere), this involves translation levels of visible parts of the desktop. Current translation statistics of main do not do a good job of representing this, because they include things like gcc error messages which are largely invisible to most users.

It would be useful to have instrumentation so that we can ask users to install a package and thereby gather information on which messages they actually see, and which ones are translated; this information can then be sent back and incorporated into the ordering presented to translators by Launchpad.