If someone is interested, other languages can be added to VisuMath. In order to do this, it is necessary to understand how the language support is organized in VisuMath. In the folder in which the program is installed, one can find the following five files related to the use of language:
All these files are ascii-files that can be opened in any text editor. The first file (VisumathLanguages.txt) contains a list of the languages that can be used. So, if a language is to be added, then the name of the language can be added. Assume for instance we want to add French. Then add Français to this list. The way the language is written is also the way it will be visible in the menu of VisuMath.
Next, two files with the name VisumathInternFrançais.txt and VisumathObjectFrançais.txt must be created, They must have the exact structure as the existing files for the other languages, but the words and sentences should be translated into French. The easiest way to construct these files is to make copies of the English files and to translate line by line to the new language. This can be a loose translation. However, make sure the total length of a translated item does not differ to much from the original one. Otherwise, items could be badly visible when VisuMath runs.
If all of this is done, then only add these files to the install directory and VisuMath will detect the new language automatically.