I get the above warning message with Moodle in the French language and the Boost theme. Here are my steps to reproduce in the latest Moodle 3.7.2+ (Build: 20190927):

1. Log in as a Moodle Administrator.
2. Install French language pack.
3. Set debugging to DEVELOPER mode
4. Switch to French language
5. Navigate to Acceuil > Mes cours > [any course name] > Participants

Near the bottom of the content area, you will see the following message:

Too much data passed as arguments to js_call_amd("core_user/participants", "init").
Generally there are better ways to pass lots of data from PHP to JavaScript, for example via Ajax, data attributes, ... . This warning is triggered if the argument string becomes longer than 1024 characters.
line 1036 of \lib\outputrequirementslib.php: call to debugging()
line 358 of \user\index.php: call to page_requirements_manager->js_call_amd()

Note: You don't need to have any students enrolled in the course and it happen in all courses.