Emdebian generates a single package for every translation of each Emdebian package, leading to a 70% reduction in installation size but a tenfold increase in the number of binary packages built from each source package. To solve this scalability problem, langupdate supports a secondary sources list and secondary apt cache so that the main apt cache can be kept as small as possible.
It looks like this "secondary apt cache" is intended to address that problem of using extra memory on things like automated security updates. Security updates would be in the main apt sources list, which would not include the translation packages. Also note that the biggest explosion of packages is really on the build machine rather than the install target -- a target device with space constraints could enable only one language at a time, even though lots of languages may be available in the package repo.