I'm usually on the other end, being a translator more often than a dev (which i'm not, to be true).
These PR have been waiting for a few years and you end up with multiple translations that conflict with one another. I don't think there is a way to automatically notify the users, but you have their emails in the php files. So you can drop them a quick mail explaining the situation. Also add a comment on the second PR and close it. something like "Until recently, this plugin was unmaintained. As the new maintainer, I accepted and committed an earlier translation, and your translation could not be merged in. If you think you can improve the current translation, please, go back to translate.dokuwiki.org. Thanks again for your time and suggestions", and I would close the PR without merging.
If that would happen to me, I would understand, and anyway be happy that there are translations for my language. ( and eventually, go check and improve the translation ;-) )