Currently the workaround is to supply a value for the second parameter: plugin_lang_get_defaulted( 'test', 'test' )
But an easy fix would be: https://github.com/mantisbt/mantisbt/pull/2278

I would expect when plugin_pluginname_test does not exist that i get 'test' back.

You are correct, this is indeed what should happen - the behavior should match that of lang_get_defaulted().

That's an oversight on my part when I implemented this function (0026747), I did not have a use case for calling the function without a default and so never tested it properly.