In the section "Support" of the PhotoFiltre Portable website are mentioned the following sentences:
Languages: PhotoFiltre Portable will automatically adjust language to match the PortableApps.com Platform. If running without the platform, you can adjust the language after running and closing the application at least once by editing the file PhotoFiltrePortableSettings.ini within the PhotoFiltrePortable\Data\settings directory. Set Language= to your preferred language (example, de for German, fr for French, etc).
But the above advices are not valid. The reason for this problem is the fact, that the appropriate language file (eg the file "StudioDE.PLG" for German) don't exists in the folder PhotoFiltre. Therefore it's necessary to paste manually the appropriate language file in the folder PhotoFiltre.