I don't see anything wrong with the structure shown on the picture, it looks like a proper JSON format.

The question is whether the beginning and the end of the file is correct... since that's what differs a proper JSON file (blabla.json) from a snippet of JavaScript code (blabla.js)!
I've seen some uneducated clients not understanding the fundamental difference between those two. These clients just see "lot of curly brackets and strings in quotation marks" and make a conclusion that it's a JSON... :-\

So, check whether your file is not in fact a piece of JavaScript code... something like this:

SC.stringsFor('en_us', {
    "_ActiveLayers" : "Active Layers",
    "_AddQuickMark" : "Add QuickMark",
    "_AddAComment" : "Add a comment",
    "_AddComment" : "Add comment"
});

or this:

strings = {
    "_ActiveLayers" : "Active Layers",
    "_AddQuickMark" : "Add QuickMark",
    "_AddAComment" : "Add a comment",
    "_AddComment" : "Add comment"
};

A correct JSON would look similar to this (i.e. without the JavaScript code piece at the beginning and the semicolon at the end):

[
  "en_us" : {
    "_ActiveLayers" : "Active Layers",
    "_AddQuickMark" : "Add QuickMark",
    "_AddAComment" : "Add a comment",
    "_AddComment" : "Add comment"
  }
]

This is the beginning of the file

Does this help?