Symptom
A translated Portfolio page such as /de/portfolio/, or a project under it, returns Page not found.
Cause
The translated archive page has a different slug from the original. The plugin maps the archive by slug, so a translated slug is not recognized as the Portfolio page.
Fix
Give the translated page the same slug as the original Portfolio page. The title can stay translated; only the slug has to match.
See Multilingual for the rest of the setup.