We’ve updated the translations, so after updating your Discourse to latest, you should be able to fix this in the UI:
You can fix your customized string by going to Customize → Text:
- Search for “js.chat.retention_reminders.dm”
- click “edit”
- click “revert changes” and confirm the question with “yes”
- click “back to search”
- Search for “js.chat.retention_reminders.dm”
- You will see 3 results, customize those strings to your liking