dict411.oxt contains DictionarySwitcher.py that implements UNO component in Python and 
the entry for it is there in META-INF/manifest.xml as application/vnd.sun.star.uno-component;type=Python.
And also the package contains ui/addons.xcu to provide the way to execute the component through the UI.

How bad is this dependency on Python for a user? Won't the dictionary work at all or the dictionary will work but the ability to switch to other variants will be broken? Will the user be given an installation error if he has chosen not to install Python?

When I tried to install the dictionary package contains DictionarySwitcher.py as 
python component on the office does not have PyUNO bridge installed, I got the 
following error: 
(com.sun.star.lang.DisposedExeption) { { { Message = "Binary URP bridge disposed during call", 
Context = (com.sun.star.uno.XInterface) @849c448 } } }

- No new package shown in the list of the installed extension.
- After restarting the office, the package is listed but disabled with "!" mark.
- Push Enable button -> message "Binary URP bridge disposed during call" twice 
  and the above message is shown again
- The extension is disabled with the "!" mark but the dictionary can be used.
- The configuration file from the package is installed and working.
- configmgr.ini is empty but backenddb.xml conains the informations about 
  the configurations provided by uninstalled extensions in the configuration backend.