Unlike many modern apps, Archicad does not use a "language pack" system for its main interface. The language is determined at the time of installation based on the specific version of the installer you used. For example, if you installed the version, you cannot switch it to INT (International/English) within the program's preferences. Step-by-Step: How to Change Your Language
– it’s the official, supported method for Archicad 26 and works reliably without reinstalling. archicad 26 change language