In your installation folder do you have a "Languages" folder? Like: "C:\Program Files\DisplayFusion\Languages"? It should have a few ".lang" files in it.
I just uninstalled DisplayFusion, removed all the folders, and re-installed Beta 21. All the folders were created correctly for me, including the "Languages" folder complete with 4 language files in it. Which ".lang". files do you have in your "Languages" folder?
If you try to double-click on the "DisplayFusion.exe" file, instead of using the shortcut, does that work? I'm wondering if perhaps your shortcut has the wrong "working directory".
I just remembered, if you start DisplayFusion with the command line parameter "/showlangerrors" it will show you why it can't load the language files. So your shortcut would be "DisplayFusion.exe /showlangerrors".
Which 2 files did you rename? Just "ru.lang" and "de-de.lang"? If so, that is strange, as DisplayFusion should be smart enough to ignore invalid files. I'll take a look at this tonight.
This beta added some new translations, which maybe triggered the files to be recognized as invalid. I have been able to reproduce this problem on my computer and it will be fixed in Beta 22, which will be released this weekend.