Normally, pdfToolbox Desktop will launch in the same language in which the the operating system is running (or English if the language in which the operating system running is not supported by pdfToolbox). It is possible to switch the language of the user interface to any other language supported by pdfToolbox Desktop. In order to do so, press the Cmd/Ctrl key while launching pdfToolbox.

A small dialog will appear, with a popup menu indicating the codes of the available languages, for example "de" for German or "fr" for French.

After selecting the desired language, simply click OK, and pdfToolbox Desktop will launch in the selected language.