Fortunately, POeditor offers a complementary instance for open source projects. From the bottom of their Pricing page:
Free Open Source Localization
Looking to translate Open Source software?
We’d love to help!
If your software has an OSI approved license,
we’ll give you unlimited strings to localize it.
Just create a project for your OS software on POEditor and fill in the Open Source Project Request in Project Settings - you’ll be on the job in no time!
We have a similar arrangement with Atlassian for Jira/Confluence and with BrowserStack. We acknowledge these contributions on the Thanks page of the developer site. If it is decided to use POeditor (or something similar) for translation support, add @michael.winkler from the Open Library Foundation and me to the discussion so we can work out the complementary usage details with the company.