The mkdocs-mdpo-plugin outputs one .po file per source file per language. It does not output any .po files for the source language (English).
TWN wants one English language .po file to use as input to it's translation memory and will output one .po file per target language which meets the configured export threshold.
The work of extracting translation units from Markdown input files to .po files and back is actually handled by https://github.com/mondeja/mdpo. The mkdocs-mdpo-plugin handles how that library is used in the mkdocs build flow.
Investigate the complexity of creating a fork or replacement of mkdocs-mdpo-plugin to better align with an established TWN workflow:
- preparation of translation dictionary as a single English language .po
- consumption of localized .po files found in git clone as a single file per target language