All pages inherit from BlogBasePage so it start there (BlogEngine.Web.Controls.BlogBasePage).
Base page in "OnLoad" method calls header.Render (BlogEngine.Web.Scripting.PageHeader).
In that render method, BE decides what language should be used for resource and puts it in the page header on top, before qnotes.js or anything else.
So now when you check page source, you should see in the page header link to language resource that looks something like:
<!-- meta and css links here -->
<!-- more links here -->
This "en-us.res.axd" is what triggers ResourceHandler.cs to be executed. You should have it registered in web.config to work.
Resource handler dynamically builds resource script (for English language in this case) and writes it to the browser with Response.Write(script).
The break point at line 118 only reached when resource not cached yet, you would need to clear browser cache to get there.