The situation varies considerably between upstream projects which have a strong and well organised upstream i18n/l10n team, such as OpenOffice.org, KDE, Gnome and similar projects, or even "smaller" projects where translations are handled through the Translation Project, and other projects for which the localisation is mostly done by direct interaction between the upstream maintainer and a variable set of volunteers.
For the first kind of project, it is recommended that Debian maintainers and translators avoid interfering with the upstream i18n/l10n resources. This means that translation updates coming through the Debian BTS should be redirected to the upstream i18n team, or to the Translation Project. In short, sending translation updates through the Debian BTS is here discouraged.
For projects where internationalisation and localisation is done autonomously, Debian maintainers can interact more closely with upstream. Most upstream maintainers may be deeply interested by the big amount of Debian resources for localisation and it is recommended to push for Debian translators to maintain as many upstream translations as possible, for such projects.
This needs a closer interaction with the upstream maintainers, for instance by requesting access to his/her revision control system or have him/her deal directly with the Debian BTS (tagging and triaging bugs). Such method has already proven to be a good method to develop a general stronger interaction with upstream, not only for localisation.