well I understand it partially. As I could understand I have to:

1.Make seperate js files for every language for example: en.js, en-us.js fr-FR.js, etc, with all key.value pairs for every language.
2. then I shopuld use these files in all other js files in my application.

but I didn`t understand that part "In the separate js file you could do a json (or ajax call) to get the translations for the culture."
Why should I do that JSON call ?

I am sorry but as you can see I don`t have it all clear. 

in these js files shold I have JSON objects like this for example:

var Strings={

confirmMessage:{
        'en/US':'Are you sure?',
        'fr/FR':'Est-ce que vous est certain?',
        ...
    },

    ...
}

 Thank You indeed for reading and trying to help me.

P.S. I am using asp.net MVC 3