That's up to the maintainer of each individual package. First and foremost, unless you're translating a package that appears only in Debian or debconf templates of a package, you really should send your translation to the upstream package where it will appear in the next upstream release. Then you need to wait for the debian package maintainer (won't hurt to ping them) to package that upstream release. Whether that package version is part of Stretch will depend on the freeze deadlines [1] (full freeze for updates to existing packages).