Ensure we don't use template strings in translatable files
Follow-up from !1014 (merged) (not the most fun hours of my life).
Quoting the main commit: ci: Prohibit template strings in translatable files
Sadly, xgettext's dealing with template strings is abysimal, so we
had to stop using them in files with translatable strings.
Make sure we don't accidentally sneak in template strings again(*)
and enforce that rule in a CI job.
(*) easy "mistake", considering how much nicer they are than
String.prototype.format()