Comment 1 for bug 787535

While I know where you are coming from (https://answers.launchpad.net/launchpad/+question/158535), the solution to this particular bug is to enable automatic export of translations to a bzr branch as well, and then merge that from time to time (or simply export to the main development branch). As such, Launchpad works in a way where it removes the need for you to manually check what translations differ since it can do the job of exporting to a version control system itself.

And even in your case of an imported external branch, I'd suggest setting up a bzr export branch which'll give you nightly exports of all the modified PO files which you can merge with (manually or otherwise) with your actual source code branch.

Basically, Launchpad already offers a solution to the problem you are describing (syncing translations back), even if it's not a solution you suggest :)

FWIW, correctly determining "what has been changed since the last import" (in the sense that it needs to be "anything that is different in Launchpad compared to the last import") would require some internal restructuring in Launchpad and is thus hard to implement. As such, I'd lean on marking this as "won't fix", but for now marking as "low" priority and tagging with "feature".