Hoo roo all,
Managed to figure this out, albeit rather messily. For all those looking in future, a Makefile won't work since it won't detect that there are targets that still need to be built.
Got strange errors when attempting to change the output directory in the Rscript line.
Instead, created build.sh that does this instead (albeit somewhat messily).
# Build English version
Rscript -e "bookdown::render_book('index.Rmd', 'all')"
# Translation Example: Build Russian Version
mkdir -p _book/ru
cd translations/ru/
Rscript -e "bookdown::render_book('index.Rmd', 'all')"
cp -r _book/* ../../_book/ru/
Any suggestions are very welcome as just threw this together after literal hours of make frustration.