It works with JNA- but, as the authors of the libvoikko plugin correctly analyzed at https://bitbucket.org/evidentsolutions/elasticsearch-analysis-voikko/src, it works for JNA in the scope of Elasticsearch core libs only, not for any plugins that use JNA. Plugins of Elasticsearch are now secondary, no longer first order citizens, i.e plugin-security.policy does not override or modify the security for ES JNA lib 
You could try to prepare a special libvoikko-enabled ES distribution by unpacking lib/elasticsearch-5.x.y.jar, modify the org/elasticsearch/bootstrap/security.policy file with the additional settings you need, and create a new jar.