Model.isIsomorphicWith() treats language tags in a case-sensitive way, which is against BCP47 spec. It is easily shown with an example:
prints false, while it clearly should print true. Related bug (which is not really a bug per se, just a trigger for this one: https://github.com/jsonld-java/jsonld-java/issues/199
See also https://issues.apache.org/jira/browse/COMMONSRDF-51 for some consideration of the language tag case.