How to disable a troublesome module that causes admin/modules to not list all modules?
I was lucky enough to have Drush available. So by simply using drush dis multilang, the module got disabled. And Eureca ... the link at admin/modules started to work again.
Even though I have not had to use another approach, this would have been my plan-b: disable the Multilang module manually in the database, via an SQL command like so:

UPDATE system SET status='0' WHERE name='multilang';

or as a variation, using PhpMyAdmin (if that's available).
If for whatever reason plan-b would not have worked either, you could consider deleting the entire module folder (for module Multilang) in the modules directory where that module got installed. That would, pretty sure, have "killed" the trouble module also. Though it would have left the module's row in the system table in some bad shape (to be repaired afterwards, probably with something like the "Registry Rebuild" module.

However, as pointed out by Molot also (merci! aka thanks!), deleting files is problematic. Because it prevents you from ever running uninstall on a module. And Registry Rebuild will probably not fix that. So think twice before using this plan-c ...