The OAI DC Ingestion Crosswalk class lacks support
for multilingual elements, for example <dc:title xml:lang='en'>, etc.
Currently DSpace (i.e. via the OAI-PMH Harvester) ignores such
language attributes because the above Java class uses a hardcoded null value for items' metadata language value.
I'm attaching a small patch that resolves the issue and I can create a Pull Request on GitHub if that is acceptable.