Can't the plugin just ignore the foreign characters and not encode them, but leave them as they are?
I'm certainly not an expert on this topic. But I found an online encoder at http://www.webutils.pl/ROTencode which just ignores the foreign characters and only changes the Latin characters.
For what it's worth, I recently wrote to the developer of the PHPEnkoder plugin, because his plugin also did not encode foreign characters correctly. He investigated and found a solution, and now his works correctly with all such characters.
I think his plugin might be using a different method of encoding though? But maybe it's still helpful to find out how he fixed it?