When the koha-common package is upgraded, it should cause koha-l10n to get upgraded as well. The koha-common package by default automatically updates translations post-install. This is redundant if the upgraded koha-l10n package hasn't also been installed.
I've been thinking about this topic on and off for a little while regarding Debian packages in general... Typically, one hard-codes required version numbers for dependencies into debian/control (or whatever process that builds it like debian/update-control in Koha). But we don't usually know the Koha/koha-l10n version until we give it to debian/build-git-snapshot (or an alternative Koha Debian package builder if you're me) which is a fairly last minute thing... We use dch -v to set the version number, which should flow through to one of the variables listed in "man deb-substvars" such as source:Version or binary:Version. That said... I don't know how well that will work for custom Debian packages which rely on the community koha-l10n package. So we might need to think of something a bit different. Possibly a custom variable in debian/control (and debian/control.in naturally), and then I think the version could be managed in debian/rules. Anyway, food for thought. Moving this to the Packaging component.