There is no UI translation available for the Cool VL Viewer, so the whole UI is exclusively in English. The UI is so different from the original (forked) viewer that I could not keep the old translations.
But if you want to provide a German translation for the UI, be my guest. All you have to to is to translate every menu file as they appear in skins/default/xui/en-us/ placing the modified files inside a new skins/default/xui/de/ directory...
Almost nothing to do... Barely 1.5Mb of files to translate !