See ticket #554 in CLARIN trac:
It matches the pattern
/c:CMD/c:Components//c:OLAC-DcmiTerms/c:subject/@olac-language
defined in facetConcepts.xml.
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[..] behaviour of the importer is that these hard-coded patterns will be ignored if the CMDI profile contains at least one element that has a matching conceptlink. This is the case here ("OLAC-DcmiTerms/language", although empty). Therefore normally this attribute is not used to fill the language facet for this profile.
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[..] convince the creator to include values in the language element in the future.
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The direct creator of these CMDI's is the harvester, so maybe something can be done there, i.e. fill the 'language' field with the value from the attribute.
(Originally submitted as https://trac.mpi.nl/ticket/4271)