Automate checking any updates required for .po files in CI
We current don’t do any sort of checks to verify that whether or not a PR changes strings and if we need to update the .po files so translations can happen for these new strings.
I already realized that some bounce processing related templates are missing from the .po files of Mailman Core. Postorius also doesn’t have new strings for labels in there, so it needs work.
We’d need to figure out a CI job which can verify this for us and possible easy ways for our contributors to update .po files when they submit MRs. This is an umbrella ticket to fix this for all Mailman subprojects currently using Weblate for translations.