Answer:
Run the program.
If you are using RFEM 5 / RSTAB 8, open the menu "Options → Program Options". In the "Program" tab, you can select another language.
Then, restart the program.
If you are using RFEM 6 / RSTAB 9, open the menu "Options → Program Options" as well. Then, select the Settings tab. You can select the language here.
You need to restart RFEM 6 / RSTAB 9 as well.
For a successful change of the language, it may be necessary to run the program as an administrator.
