di has an odd build system. Under the hood, it uses CMake, but that is
wrapped in another layer of make. This extra layer prevents debhelper
from properly setting up a CMake cross build and therefore di always
builds for the build architecture.

I suggest skipping the wrapping and driving CMake directly. The downside
is that this has to be kept in sync with upstream changes to the
wrapping Makefile. The good side is that once using CMake directly,
debhelper automatically handles a few things correctly - including cross
builds.

I'm attaching a patch for your convenience. Note that I list locales and
examples in debian/not-installed. That is what the packaging currently
does, but I suggest that locales should be installed into the di package
and examples should be installed into the libdi-dev package.

Helmut