Beginning with Workbench 2.1 localisation was introduced. This means, you had a prefs program where you could choose one of the installed languages and the Workbench immediately changed its language. Most programs supporting locale.library do the same, some need to be stopped and restarted, but you don't need to install another Workbench version, you don't even need to reboot in order to switch languages.
Installing new languages just means to copy the catalog files to the Locale directory of the boot partition.
Earlier Workbench versions (2.0 and below) only existed in english. Only the keyboard layout could be changed.