Make plural alternatives as plural-forms messages about data.table

[MichaelChirico]

Thanks for tracking these down! Yes please switch to ngettext(). So far they haven't been sussed out because Mandarin doesn't really have plural forms, so translating there is easy without using ngettext().

[MichaelChirico]

For the C messages, we don't have any ngettext() usage there yet, so please ensure we are properly set up to start doing plural translations there:

1. Make sure the package compiles and can pass tests, obviously :)
2. Make sure the recommended translation workflow is picking these new strings up

data.table/.dev/CRAN_Release.cmd

Lines 11 to 26 in de0cf94

	xgettext -o /dev/stdout ./*.c \
	--keyword=Rprintf --keyword=error --keyword=warning --keyword=STOP --keyword=DTWARN --keyword=Error --keyword=DTPRINT --keyword=snprintf:3
	## (b) Update R template file: src/R-data.table.pot
	## NB: this relies on R >= 4.0 to remove a bug in update_pkg_po
	Rscript -e "tools::update_pkg_po('.')"
	# 2) Open a PR with the new templates & contact the translators
	# * zh_CN: @hongyuanjia
	## Translators to submit commits with translations to this PR
	## [or perhaps, if we get several languages, each to open
	## its own PR and merge to main translation PR]
	## 3) Check validity with tools::checkPoFiles("zh_CN")
	## 4) Compile the new .mo binary files with potools::po_compile()

At a glance, I don't think it will -- I don't see ngettext() mentioned there.