Strings provided by Python scripted and CLI modules, strings that are dynamically added by C++ loadable modules and not marked as translatable, and strings in extensions are not available for translation yet. See a bit more information here: https://github.com/Slicer/Slicer/issues/6177
We are in the process of hiring an engineer dedicated to internationalization (funded by CZI Essential Open-Source Software grant), so it will probably take a few months until all these missing strings become available for translation. In the meantime, if any software developer had time to help out with these tasks then we would be very happy to collaborate.